Add Gtk::LockButton

	* .gitignore: Ignore gtk/gtkmm/lockbutton.[cc|h]
	* docs/reference/images/widgets/lockbutton*: New files
	containing lock button widget states.
	* gtk/src/filelist.am: Add gtk/src/lockbutton.hg
	* gtk/src/lockbutton.[ccg|hg]: New files.
	* tools/m4/convert_gtk.m4: Needed conversions for
	Gio::Permission.