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'm not really sure either. I think `gtk_init` might be a no-op if called twice. And those fixes also let you call `gtk_main` multiple times, across multiple tasks -- although julia's `COPY_STACKS` corrupts the stack if you try that :(