Add a menu to Debug mode that switches perspectives

Set a menu for switching perspectives on the debug mode icon. When icons
& text are shown for the modes, there is an additional arrow to the
right of the mode that can be clicked to open the menu. This
functionality was added in 4.1 for that purpose, finally use it.
Do not show and arrow in "Icons Only" mode, so it stays small, but make
it available on right-click as well.

Change-Id: If19877b5231b7f01f0b3fd98cf65e8eaf9da1a5f
Reviewed-by: hjk <hjk@qt.io>

src/plugins/debugger/debuggerplugin.cpp

@@ -547,6 +547,8 @@ public:
         mainWindow->addSubPerspectiveSwitcher(EngineManager::engineChooser());
 
         setWidget(splitter);
+
+        setMenu(DebuggerMainWindow::perspectiveMenu());
     }
 
     ~DebugMode() { delete widget(); }