This decouples wlr_output_enable and the wl_global.
The previously internal functions wlr_output_(destroy/create)_global are
exposed and used automatically in the wlr_output_layout to create/tear
down the global.
The compositor can handle them itself if it wants to, but I think this
is the right moment to create/destroy the wl_output when the
wlr_output_layout is used.