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.
Ok after fetching your recent changes it now works to call `Gtk.run(app)` as the last statement.

But I am still not sure how `__init__` should look like. The call to `gtk_init` should actually be removed when using gtkapplication.