Remove ::update virtual function from Gui Base class.
 - We can probably remove this class all-together now.
Remove gt::Core loop
Change Gtk to handle its own event loop
Link Gtk::Main::quit() with GtkMainWindow's destroy signal
Remove Gtk destructor and raw GtkMainWindow pointer
Run GtkMainWindow on the heap.

 - Automatically cleaned up after Gtk shutdown
 - Fixes Glibmm critical assertion error
Remove Gtk::Main::quit() as it is no longer required to exit safely
Merge pull request #18 from truNEET/wip/loop

Let Gtk handle its own event loop