[Qt] New status bar Unit Display Control and related changes.

- New status bar control shows the current Unit of Display.
  When clicked (left,or right button) it shows a context menu
  that allows the user to switch the current Unit of Display (BTC, mBTC, uBTC)
- Recent Requests and Transaction Table headers are now updated when
  unit of display is changed, because their "Amount" column now displays the
  current unit of display.
- Takes care of issue #3970 Units in transaction export csv file.
- Small refactors for reusability.
- Demo Video https://www.youtube.com/watch?v=wwcr0Yh68go&list=UUG3jF2hgofmLWP0tRPisQAQ
- changes after Diapolo's feedback. Have not been able to build after last pool, issues with boost on MacOSX, will test on Ubuntu these changes.
- removed return statement on switch
- renamed onDisplayUnitsChanged(int) to updateDisplayUnit(int)
- now getAmountColumnTitle(int unit) takes a simple unit parameter. moved to BitcoinUnits.

src/qt/transactionview.cpp

@@ -309,7 +309,7 @@ void TransactionView::exportClicked()
     writer.addColumn(tr("Type"), TransactionTableModel::Type, Qt::EditRole);
     writer.addColumn(tr("Label"), 0, TransactionTableModel::LabelRole);
     writer.addColumn(tr("Address"), 0, TransactionTableModel::AddressRole);
-    writer.addColumn(tr("Amount"), 0, TransactionTableModel::FormattedAmountRole);
+    writer.addColumn(BitcoinUnits::getAmountColumnTitle(model->getOptionsModel()->getDisplayUnit()), 0, TransactionTableModel::FormattedAmountRole);
     writer.addColumn(tr("ID"), 0, TransactionTableModel::TxIDRole);
 
     if(!writer.write()) {