[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/bitcoinunits.cpp

@@ -169,6 +169,16 @@ bool BitcoinUnits::parse(int unit, const QString &value, qint64 *val_out)
     return ok;
 }
 
+QString BitcoinUnits::getAmountColumnTitle(int unit)
+{
+    QString amountTitle = QObject::tr("Amount");
+    if (BitcoinUnits::valid(unit))
+    {
+        amountTitle += " ("+BitcoinUnits::name(unit) + ")";
+    }
+    return amountTitle;
+}
+
 int BitcoinUnits::rowCount(const QModelIndex &parent) const
 {
     Q_UNUSED(parent);