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.
ah, then perhaps not. it might exit differently because it isn't the toplevel gtk_main (so gtk_quit doesn't do quite the right thing). but that primarily matters if being in the toplevel gtk_main would block program termination, but in our case, it doesn't