[GUI] Add checkboxSubtractFeeFromAmount checkbox in sendmultirow

Visible when single recipient. Its checked state returns whether or not
to subtract the fee from the recipient amount.

SendMultiRow::toggleSubtractFeeFromAmount is not connected for now.
It will be connected with the contextual menu, to set per-recipient
subtract-fee-from-amount, when there are multiple rows.

src/qt/pivx/forms/sendmultirow.ui

@@ -347,6 +347,16 @@ padding:0px;</string>
          <property name="bottomMargin">
           <number>0</number>
          </property>
+         <item>
+          <widget class="QCheckBox" name="checkboxSubtractFeeFromAmount">
+           <property name="toolTip">
+            <string>The fee will be deducted from the amount being sent. The recipient will receive less PIV than you enter in the amount field. If multiple recipients are selected, the fee is split equally.</string>
+           </property>
+           <property name="text">
+            <string>Subtract fee from amount</string>
+           </property>
+          </widget>
+         </item>
          <item>
           <spacer name="horizontalSpacer">
            <property name="orientation">

src/qt/pivx/sendconfirmdialog.cpp

@@ -183,14 +183,26 @@ void TxDetailDialog::setData(WalletModel *_model, WalletModelTransaction* _tx)
     this->model = _model;
     this->tx = _tx;
     CAmount txFee = tx->getTransactionFee();
-    CAmount totalAmount = tx->getTotalTransactionAmount() + txFee;
 
     // inputs label
     CTransactionRef walletTx = tx->getTransaction();
     setInputsType(walletTx);
 
+    bool fSubtractFee = false;
+    const QList<SendCoinsRecipient>& recipients = tx->getRecipients();
+    for (const SendCoinsRecipient& rec : recipients) {
+        if (rec.fSubtractFee) {
+            fSubtractFee = true;
+            break;
+        }
+    }
+
+    CAmount totalAmount = tx->getTotalTransactionAmount();
+    if (!fSubtractFee) totalAmount += txFee;
+
     ui->textAmount->setText(BitcoinUnits::formatWithUnit(nDisplayUnit, totalAmount, false, BitcoinUnits::separatorAlways) + " (Fee included)");
-    int nRecipients = tx->getRecipients().size();
+
+    int nRecipients = recipients.size();
     if (nRecipients == 1) {
         const SendCoinsRecipient& recipient = tx->getRecipients().at(0);
         if (recipient.isP2CS) {

src/qt/pivx/sendmultirow.cpp

@@ -225,6 +225,7 @@ SendCoinsRecipient SendMultiRow::getValue()
     recipient.amount = getAmountValue();
     auto dest = Standard::DecodeDestination(recipient.address.toStdString());
     recipient.isShieldedAddr = boost::get<libzcash::SaplingPaymentAddress>(&dest);
+    recipient.fSubtractFee = getSubtractFeeFromAmount();
     return recipient;
 }
 
@@ -263,6 +264,11 @@ int SendMultiRow::getNumber()
     return number;
 }
 
+bool SendMultiRow::getSubtractFeeFromAmount() const
+{
+    return ui->checkboxSubtractFeeFromAmount->isChecked();
+}
+
 void SendMultiRow::setAddress(const QString& address)
 {
     ui->lineEditAddress->setText(address);
@@ -274,6 +280,12 @@ void SendMultiRow::setAmount(const QString& amount)
     ui->lineEditAmount->setText(amount);
 }
 
+void SendMultiRow::toggleSubtractFeeFromAmount()
+{
+    bool old = ui->checkboxSubtractFeeFromAmount->isChecked();
+    ui->checkboxSubtractFeeFromAmount->setChecked(!old);
+}
+
 void SendMultiRow::setAddressAndLabelOrDescription(const QString& address, const QString& message)
 {
     QString label = walletModel->getAddressTableModel()->labelForAddress(address);

src/qt/pivx/sendmultirow.h

@@ -50,11 +50,13 @@ class SendMultiRow : public PWidget
     void setAmount(const QString& amount);
     void setAddressAndLabelOrDescription(const QString& address, const QString& message);
     void setFocus();
+    void toggleSubtractFeeFromAmount();
 
     QRect getEditLineRect();
     int getEditHeight();
     int getEditWidth();
     int getMenuBtnWidth();
+    bool getSubtractFeeFromAmount() const;
 
     // Return true if memo was set and false if it was cleared.
     bool launchMemoDialog();

src/qt/walletmodel.cpp

@@ -488,7 +488,7 @@ WalletModel::SendCoinsReturn WalletModel::prepareTransaction(WalletModelTransact
                 // Regular P2PK or P2PKH
                 scriptPubKey = GetScriptForDestination(out);
             }
-            vecSend.emplace_back(scriptPubKey, rcp.amount, false);
+            vecSend.emplace_back(scriptPubKey, rcp.amount, rcp.fSubtractFee);
 
             total += rcp.amount;
         }
@@ -599,9 +599,9 @@ OperationResult WalletModel::PrepareShieldedTransaction(WalletModelTransaction*
                                                         const CCoinControl* coinControl)
 {
     // Load shieldedAddrRecipients.
-    bool fSubtractFeeFromAmount{false};
     std::vector<SendManyRecipient> recipients;
     for (const auto& recipient : modelTransaction->getRecipients()) {
+        bool fSubtractFeeFromAmount = recipient.fSubtractFee;
         if (recipient.isShieldedAddr) {
             auto pa = KeyIO::DecodeSaplingPaymentAddress(recipient.address.toStdString());
             if (!pa) return errorOut("Error, invalid shielded address");

src/qt/walletmodel.h

@@ -70,6 +70,9 @@ class SendCoinsRecipient
     // Quick flag to not have to check the address type more than once.
     bool isShieldedAddr{false};
 
+    // Whether to subtract the tx fee from this recipient
+    bool fSubtractFee{false};
+
     // Amount
     CAmount amount{0};
     // If from a payment request, this is used for storing the memo