A weston_output available to the compositor should always be either in
the list of pending outputs or the list of enabled outputs. Let
weston_compositor_add_output() and weston_compositor_remove_output()
handle the moves between the lists.

This way weston_output_enable() does not need to remove and
oops-it-failed-add-it-back. weston_output_disable() does not need to
manually re-add the output back to the pending list.

To make everything nicely symmetric and fix any unbalancing caused by
this:
- weston_output_destroy() explicitly wl_list_remove()s
- weston_compositor_add_pending_output() first removes then inserts, as
we have the assumption that the link is always valid, even if empty.

Update the documentations, too.

v2:
- talk about "list of enabled outputs"
- keep wl_list_remove in weston_compositor_remove_output in its old
  place

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Armin Krezović <krezovic.armin@gmail.com>