[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.cpp

@@ -21,7 +21,9 @@ RecentRequestsTableModel::RecentRequestsTableModel(CWallet *wallet, WalletModel
         addNewRequest(request);
 
     /* These columns must match the indices in the ColumnIndex enumeration */
-    columns << tr("Date") << tr("Label") << tr("Message") << tr("Amount");
+    columns << tr("Date") << tr("Label") << tr("Message") << getAmountTitle();
+
+    connect(walletModel->getOptionsModel(), SIGNAL(displayUnitChanged(int)), this, SLOT(updateDisplayUnit()));
 }
 
 RecentRequestsTableModel::~RecentRequestsTableModel()
@@ -101,6 +103,24 @@ QVariant RecentRequestsTableModel::headerData(int section, Qt::Orientation orien
     return QVariant();
 }
 
+/** Updates the column title to "Amount (DisplayUnit)" and emits headerDataChanged() signal for table headers to react. */
+void RecentRequestsTableModel::updateAmountColumnTitle()
+{
+    columns[Amount] = getAmountTitle();
+    emit headerDataChanged(Qt::Horizontal,Amount,Amount);
+}
+
+/** Gets title for amount column including current display unit if optionsModel reference available. */
+QString RecentRequestsTableModel::getAmountTitle()
+{
+    QString amountTitle = tr("Amount");
+    if (this->walletModel->getOptionsModel() != NULL)
+    {
+        amountTitle += " ("+BitcoinUnits::name(this->walletModel->getOptionsModel()->getDisplayUnit()) + ")";
+    }
+    return amountTitle;
+}
+
 QModelIndex RecentRequestsTableModel::index(int row, int column, const QModelIndex &parent) const
 {
     Q_UNUSED(parent);
@@ -185,6 +205,11 @@ void RecentRequestsTableModel::sort(int column, Qt::SortOrder order)
     emit dataChanged(index(0, 0, QModelIndex()), index(list.size() - 1, NUMBER_OF_COLUMNS - 1, QModelIndex()));
 }
 
+void RecentRequestsTableModel::updateDisplayUnit()
+{
+    updateAmountColumnTitle();
+}
+
 bool RecentRequestEntryLessThan::operator()(RecentRequestEntry &left, RecentRequestEntry &right) const
 {
     RecentRequestEntry *pLeft = &left;