Use _NET_VIRTUAL_ROOTS for querying virtual roots

[Caellian]

VRootWindowOfScreen which was frequently called from event propagation code used to query window properties (XGetWindowProperty) on every event, for all top level windows. This PR addresses this by querrying only _NET_VIRTUAL_ROOTS atom once per call.

Fixes #1852.

Testing

• Behaves correctly on WMs that use virtual roots (swm, tvtwm, amiwm, enlightenment)
  • Test by checking whether _NET_CURRENT_DESKTOP index matches current vroot index in _NET_VIRTUAL_ROOTS list.

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[Caellian]

Can't get any WMs that use virtual roots to compile and most aren't maintained anymore. Except enlightenment, which doesn't use virtual roots anymore. I'll assume _NET_CURRENT_DESKTOP index matches current vroot index in _NET_VIRTUAL_ROOTS list and merge this as the current implementation is horribly slow.

It will be less pressure to fix vroots not working as it affects far fewer people, and when they create an issue I can ask them to provide details I'm missing atm.