decouples outputenable state and wl_output global

Created by: Ongy

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.

Examples are not updates as they either use wlr_output_layout or won't make use of the wl_output either way.