[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.h

@@ -52,6 +52,8 @@ public:
     int rowCount(const QModelIndex & parent = QModelIndex()) const;
     QVariant data(const QModelIndex & index, int role = Qt::DisplayRole) const;
     bool setData(const QModelIndex & index, const QVariant & value, int role = Qt::EditRole);
+    /** Updates current unit in memory, settings and emits displayUnitChanged(newUnit) signal */
+    void setDisplayUnit(const QVariant &value);
 
     /* Explicit getters */
     bool getMinimizeToTray() { return fMinimizeToTray; }