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.
Not sure. Have not tested this yet. The documentation says one should not call `gtk_main` when using GtkApplication.