Remove custom OSD brightness popup, use libnotify instead

The custom rendered OSD popup to indicate the current brightness
state whenever it changed looked nice but it is better handled in
the notification daemon. E.g. we could include an extension hint
in the notification to have xfce4-notifyd render certain
notifications (e.g. brightness, volume changes) in a different style
than regular notifications.

This commit simplifies the whole brightness notification logic
quite a bit.

Signed-off-by: Jannis Pohlmann <jannis@xfce.org>