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.
i checked the glib code again, and realized the concerning code i had seen in the `2.34.1` source has been removed in `2.40.0` (i'm running the newest version, but had older code lying around). now that Keno has fixed the backtrace printing, could you run it again and see if it prints a more meaningful stack trace?