Gtk::Main is deprecated, but the new solution, instantiating a
Gtk::Application object does not match our use case, since we handle
all application concerns already and just need a Gtk main loop to run.
Anyway, it became clear that the "main object" will be the new UiManager.
As a first step, I've now moved the (deprecated) Gtk::Main object
down there. Next step (planned) will be to inherit from Gio::Application
and clone some functionality from Gtk::Application
