na-tray-child.[ch]: Add na_tray_child_set_composited() to allow the
application to modify the behavior where gdk_window_set_composited(TRUE)
is called for icons with alpha. Rename na_tray_child_is_composited() to
na_tray_child_has_alpha() to better reflect the meaning and fix things
so that it can be called at any time and not only after the icon is
realized.
na-tray.c: Adapt to na_tray_child_is_composited() rename.
http://bugzilla.gnome.org/show_bug.cgi?id=579454
58e718ac0c