overlays: encapsulate global overlays state

Introduce vwm_overlays_t and create/destroy functions, use in vwm_startup()
and vwm_shutdown().  Supply to methods operating on the global overlays
state vwm_overlays_update(), vwm_overlays_rate_increase(),
vwm_overlays_rate_decrease().

This is a fairly minimal adoption of these changes with vwm_t still being
supplyed to the overlay functions.

A future commit will further cleanup the interactions and cease all
knowledge of vwm_t in overlays.c, but for now everything overlay-oriented
still accesses the overlays_t instance via vwm_t.  Instead of supplying the
vwm_t to vwm_overlays_create() the bare vwm_xserver_t will be supplied, as
this is the future shared component across vmon and vwm (in addition to
overlays).

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