hwmixvolume: switch to GTK+ 3.0 and GLib 2.0

hwmixvolume: switch to GTK+ 3.0 and GLib 2.0

This replaces VBox and HBox with Grid (using Gtk.Orientation), HScale
with Scale, creates labels with mnemonics, set hexpand and vexpand
properly, use the correct enum container classes, use the correct getter
for size request, and finally update to the correct GLib watch function.

Signed-off-by: Emmanuel Gil Peyrot <linkmauve@linkmauve.fr>
Reviewed-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>