[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/recentrequeststablemodel.h

@@ -91,12 +91,18 @@ public:
 
 public slots:
     void sort(int column, Qt::SortOrder order = Qt::AscendingOrder);
+    void updateDisplayUnit();
 
 private:
     WalletModel *walletModel;
     QStringList columns;
     QList<RecentRequestEntry> list;
     int64_t nReceiveRequestsMaxId;
+
+    /** Updates the column title to "Amount (DisplayUnit)" and emits headerDataChanged() signal for table headers to react. */
+    void updateAmountColumnTitle();
+    /** Gets title for amount column including current display unit if optionsModel reference available. */
+    QString getAmountTitle();
 };
 
 #endif