Merge bitcoin-core/gui#408: Add missing mnemonics in menu bar options

7c33e3a qt: Add missing mnemonics in menu bar options (Shashwat)

Pull request description:

  Since ElementsProject#362 we have defaulted to add mnemonic shortcuts for the context menus.
  The Window -> Minimize option and File -> Load PSBT from clipboard were hitherto missing a mnemonic shortcut. This PR adds mnemonic shortcuts for them

  Changes introduced in this PR:
  | Master | PR |
  | ----------| ---- |
  | ![Screenshot from 2021-08-23 23-10-07](https://user-images.githubusercontent.com/85434418/130494098-c65ec9da-c3f1-4243-9b3d-0c87cb677825.png) | ![Screenshot from 2021-08-23 23-08-41](https://user-images.githubusercontent.com/85434418/130494083-849ffd14-05e9-4a6d-bdc3-b3e55b8a8861.png)|
  |![Screenshot from 2021-08-23 23-10-21](https://user-images.githubusercontent.com/85434418/130494233-1b2e8838-c5d4-48a8-9abf-a4acc8d6146c.png)|![Screenshot from 2021-08-23 23-09-00](https://user-images.githubusercontent.com/85434418/130494181-dcdbf119-a2c6-469d-a6c9-3021506ab40b.png)|

ACKs for top commit:
  jarolrod:
    tACK 7c33e3a
  hebasto:
    ACK 7c33e3a, tested on Linux Mint 20.2 (Qt 5.12.8).

Tree-SHA512: 32f201ae7716b19ca123856292f8bfa0d805f6c7271ac1b5e08eff6b95017443754c8a76e8396ccca1869a57384e11016cbd99d63ccdd2fae6da4eaf3ae32298

src/qt/bitcoingui.cpp

@@ -329,7 +329,7 @@ void BitcoinGUI::createActions()
     verifyMessageAction->setStatusTip(tr("Verify messages to ensure they were signed with specified Bitcoin addresses"));
     m_load_psbt_action = new QAction(tr("&Load PSBT from file…"), this);
     m_load_psbt_action->setStatusTip(tr("Load Partially Signed Bitcoin Transaction"));
-    m_load_psbt_clipboard_action = new QAction(tr("Load PSBT from clipboard…"), this);
+    m_load_psbt_clipboard_action = new QAction(tr("Load PSBT from &clipboard…"), this);
     m_load_psbt_clipboard_action->setStatusTip(tr("Load Partially Signed Bitcoin Transaction from clipboard"));
 
     openRPCConsoleAction = new QAction(tr("Node window"), this);
@@ -486,7 +486,7 @@ void BitcoinGUI::createMenuBar()
 
     QMenu* window_menu = appMenuBar->addMenu(tr("&Window"));
 
-    QAction* minimize_action = window_menu->addAction(tr("Minimize"));
+    QAction* minimize_action = window_menu->addAction(tr("&Minimize"));
     minimize_action->setShortcut(QKeySequence(Qt::CTRL + Qt::Key_M));
     connect(minimize_action, &QAction::triggered, [] {
         QApplication::activeWindow()->showMinimized();