Introduce runAsync() to run main loops without blocking module resolution

All modules are promises. If Gtk.Application.run() or
GLib.MainLoop.run() is called within a module it will block all other
promises as run() is a synchronous, blocking function. To work around
this there is now a setMainLoopHook function exposed which runs a
callback after module resolution is complete. This allows APIs to
"install" a mainloop asynchronously.

For Gio.Application and GLib.MainLoop there are now runAsync() versions
of their run() functions. runAsync() returns a Promise which will
resolve once the mainloop has exited.

Fixes #468