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.
    tobias-knopps-macbook-2:test knopp$ julia  gtkapplication_tests.jl 
    **
    GLib:ERROR:gmain.c:3069:g_main_dispatch: assertion failed: (current->dispatching_sources == &current_source_link)
    
    signal (6): Abort trap: 6
    __pthread_kill at /usr/lib/system/libsystem_kernel.dylib (unknown line)
    Abort trap: 6

But if `gtk_main` is scheduled first and afterwards `Gtk.run(app)` is called, do you really expect that this would work.