[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.h

@@ -54,6 +54,8 @@ public:
     static QString formatWithUnit(int unit, qint64 amount, bool plussign=false);
     //! Parse string to coin amount
     static bool parse(int unit, const QString &value, qint64 *val_out);
+    //! Gets title for amount column including current display unit if optionsModel reference available */
+    static QString getAmountColumnTitle(int unit);
     ///@}
 
     //! @name AbstractListModel implementation