[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/optionsmodel.cpp

@@ -308,9 +308,7 @@ bool OptionsModel::setData(const QModelIndex & index, const QVariant & value, in
             break;
 #endif
         case DisplayUnit:
-            nDisplayUnit = value.toInt();
-            settings.setValue("nDisplayUnit", nDisplayUnit);
-            emit displayUnitChanged(nDisplayUnit);
+            setDisplayUnit(value);
             break;
         case DisplayAddresses:
             bDisplayAddresses = value.toBool();
@@ -356,11 +354,24 @@ bool OptionsModel::setData(const QModelIndex & index, const QVariant & value, in
             break;
         }
     }
+
     emit dataChanged(index, index);
 
     return successful;
 }
 
+/** Updates current unit in memory, settings and emits displayUnitChanged(newUnit) signal */
+void OptionsModel::setDisplayUnit(const QVariant &value)
+{
+    if (!value.isNull())
+    {
+        QSettings settings;
+        nDisplayUnit = value.toInt();
+        settings.setValue("nDisplayUnit", nDisplayUnit);
+        emit displayUnitChanged(nDisplayUnit);
+    }
+}
+
 bool OptionsModel::getProxySettings(QNetworkProxy& proxy) const
 {
     // Directly query current base proxy, because