WIP: GtkApplication Experiments

Ok I had a look at `GtkApplication` and got a simple example running including a global menu on OSX. Yay.

But this raises questions how to move forward. Much of the functionality around `GtkApplication` is implemented in `GIO`. So is it possible to autogenerate the accessors for `GIO` types as well?

To get the menu actually working one has to implement the `GAction` interfaces. All this looks a little complicated and I wonder if one could define this using the regular `GtkMenu` in code (without `GtkBuilder`) with regular callbacks.
Indeed, the `gtk_init` does not seem to hurt when using gtkapplication. when scheduling `gtk_main` in `__init__` and afterwards calling `Gtk.run(app)` I get a crash though

    signal (6): Abort trap: 6
    ??? at ???:202552579
    ??? at ???:202544405
    ??? at ???:-1907329622
    Abort trap: 6

So it seems that gtk_init can stay but gtk_main should be removed and the user simply has to call it.