Refactor GUI destruction

[franzlst]

I started cleaning the GUI destruction process in #433. Yet, there are still some aspects that I don't like:

• The prepare_destruction method of the MainWindowController calls self.destroy(), which then recursively destroys the whole GUI. This is not a prepare, but a full destroy. Especially, because this also triggers MenuBarController's on_destroy, which stops the gtk main loop.
• Many functionalities (see on_quit_activate, on_delete_event, on_destroy) is handled in the MenuBarController. I think, the GUI should work without this controller. Thus, this code should be moved probably to the MainWindowController

Originally created by @franzlst ([email protected]) at 2017-11-22 08:56:28+00:00 (moved from RMC internal repository)