[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/bitcoingui.h

@@ -9,12 +9,16 @@
 #include "config/bitcoin-config.h"
 #endif
 
+#include <QLabel>
 #include <QMainWindow>
 #include <QMap>
+#include <QMenu>
+#include <QPoint>
 #include <QSystemTrayIcon>
 
 class ClientModel;
 class Notificator;
+class OptionsModel;
 class RPCConsole;
 class SendCoinsRecipient;
 class WalletFrame;
@@ -22,9 +26,13 @@ class WalletModel;
 
 class CWallet;
 
+class UnitDisplayStatusBarControl;
+
 QT_BEGIN_NAMESPACE
 class QAction;
 class QLabel;
+class QMenu;
+class QPoint;
 class QProgressBar;
 class QProgressDialog;
 QT_END_NAMESPACE
@@ -69,6 +77,7 @@ private:
     ClientModel *clientModel;
     WalletFrame *walletFrame;
 
+    UnitDisplayStatusBarControl *unitDisplayControl;
     QLabel *labelEncryptionIcon;
     QLabel *labelConnectionsIcon;
     QLabel *labelBlocksIcon;
@@ -198,4 +207,32 @@ private slots:
     void showProgress(const QString &title, int nProgress);
 };
 
+class UnitDisplayStatusBarControl : public QLabel
+{
+    Q_OBJECT
+
+public:
+    explicit UnitDisplayStatusBarControl();
+    /** Lets the control know about the Options Model (and its signals) */
+    void setOptionsModel(OptionsModel *optionsModel);
+
+protected:
+    /** So that it responds to left-button clicks */
+    void mousePressEvent(QMouseEvent *event);
+
+private:
+    OptionsModel *optionsModel;
+    QMenu* menu;
+    /** Shows context menu with Display Unit options by the mouse coordinates */
+    void onDisplayUnitsClicked(const QPoint& point);
+    /** Creates context menu, its actions, and wires up all the relevant signals for mouse events. */
+    void createContextMenu();
+
+private slots:
+    /** When Display Units are changed on OptionsModel it will refresh the display text of the control on the status bar */
+    void updateDisplayUnit(int newUnits);
+    /** Tells underlying optionsModel to update its current display unit. */
+    void onMenuSelection(QAction* action);
+};
+
 #endif // BITCOINGUI_H