Addframe highlighting support

Addframe highlighting support

	* ../src/pgtkterm.c:
	(pgtk_focus_frame, set_opacity_recursively, x_set_frame_alpha)
	(frame_highlight, frame_unhighlight, pgtk_frame_rehighlight)
	(XTframe_rehighlight, x_new_focus_frame, pgtk_create_terminal):

frame の highlight に対応。

とりあえず opacity のみ対応。