[GUI] Start MN update timer on launch and on governance page.

Determining if a proposal is over budget requires an accurate count of
the number of MNs active on the network. To get this, start the MN
update timer on launch as well as when activating the DAO page.

src/qt/clientmodel.cpp

@@ -50,6 +50,7 @@ ClientModel::ClientModel(OptionsModel* optionsModel, QObject* parent) : QObject(
 
     pollMnTimer = new QTimer(this);
     connect(pollMnTimer, &QTimer::timeout, this, &ClientModel::updateMnTimer);
+    startMasternodesTimer();
 
     subscribeToCoreSignals();
 }

src/qt/pivx/governancewidget.cpp

@@ -270,6 +270,7 @@ void GovernanceWidget::loadWalletModel()
 
 void GovernanceWidget::showEvent(QShowEvent *event)
 {
+    clientModel->startMasternodesTimer();
     tryGridRefresh(true); // future: move to background worker
     if (!refreshTimer) refreshTimer = new QTimer(this);
     if (!refreshTimer->isActive()) {
@@ -281,6 +282,7 @@ void GovernanceWidget::showEvent(QShowEvent *event)
 void GovernanceWidget::hideEvent(QHideEvent *event)
 {
     refreshTimer->stop();
+    clientModel->stopMasternodesTimer();
 }
 
 void GovernanceWidget::wheelEvent(QWheelEvent* event)