add GtkAction functionality
add GtkAction, GtkActionGroup functionality
Merge branch 'gtk-action' of https://github.com/d2r2/go-gtk into gtk-action

Conflicts:
	example/action/action.go
	gtk/gtk.go
Merge branch 'gtk-action' of https://github.com/d2r2/go-gtk into gtk-action

Conflicts:
	example/action/action.go
	gtk/gtk.go
Merge branch 'gtk-action' of https://github.com/d2r2/go-gtk into gtk-action
Merge branch 'gtk-action' of https://github.com/d2r2/go-gtk into gtk-action
Merge branch 'gtk-action' of https://github.com/d2r2/go-gtk into gtk-action
Merge branch 'gtk-action' of https://github.com/d2r2/go-gtk into gtk-action
Merge branch 'gtk-action' of https://github.com/d2r2/go-gtk into gtk-action
Merge branch 'gtk-action' of https://github.com/d2r2/go-gtk into gtk-action
Merge branch 'gtk-action' of github.com:d2r2/go-gtk into gtk-action
Add GtkAction functionality.
Demo app examples/action/action.go
Add GtkAction functionality and some auxiliary code.
Find demo in example/action/action.go