ApplicationWindow: update paste sensitivity according to clipboard

I think it is a better implementation than in gedit. We look at all the
target atoms supported by GtkTextBuffer.

gedit connects to the GtkClipboard::owner-change signal when the
GtkWindow is realized. But I don't think this is still necessary with
GTK+ 3. The doc of gtk_widget_get_clipboard() says: “@widget must have a
GdkDisplay associated with it, so must be attached to a toplevel
window.”, but here @widget *is* a toplevel window, so I think the
GdkDisplay is already associated. I've tested and in
tepl_application_window_constructed(), the GtkApplicationWindow is not
realized but the GtkClipboard isn't NULL. So I think everything is fine.