Add GtkApplication

Add GtkApplication

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