Issue #1432: Popup menu in "Open Database"

A popup menu has been added in the "Open Database" button. A normal click
selects the "Open Database..." action as before. A delayed click opens the
popup menu for selecting either the "Open Database..." action or the
"Open Database Read Only..." action.

src/MainWindow.h

@@ -145,6 +145,7 @@ private:
 
     QMenu* popupTableMenu;
     QMenu* recentFilesMenu;
+    QMenu* popupOpenDbMenu;
     QMenu* popupSaveSqlFileMenu;
     QMenu* popupSaveSqlResultsMenu;
     QMenu* popupSaveFilterAsMenu;