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.
yes. this indicates you are running on a version of glib prior to 2.40.0 with COPY_STACKS in julia. it also potentially indicates that I'm returning from the jl_yield callbacks in the wrong order, but I thought i fixed that in https://github.com/JuliaLang/Gtk.jl/commit/0a63ea7ec14e35abccbbc1f2e013b5fe7f1590ef#diff-8c2b506e7179e52e816713686d731068R248 (although rereading that code, i see i'm missing a try/catch block)