This is a work in progress to stub out an application class. The
primary goal is to provide a mechanism for applications to export
GtkActions, and there is a standard "Quit" action.
This is based on GApplication.
Future work:
* Add a way to say "This is my application menubar", which gets
put into all toplevel windows on non-OS-X, and into the top
on OS X.
* Support session management.
* Support application settings.
https://bugzilla.gnome.org/show_bug.cgi?id=127958
