Initial commit of map zone library
Add support to rotate zones

Admins can rotate zones using their mouse while holding +use (e). If
+speed (shift) is pressed too, the x and z axis switch control.
Add sm_mapzone_debug convar

Can be used to show all zones by default.
Don't count deleted zones as name duplicates
Added zone clusters

You can join several groups into one cluster and have the enter and
leave forward called only once while the player stays within the zones
of the cluster.

That way you can cover more complicated forms of zones as one.
Kill triggers on plugin unload
Add option to set cluster for existing zones

Zones can be added to a cluster if they are in none or added to one.
Add MapZone_SetMenuBackAction native

Plugins can make the transition from the map zone menus back to their
own menu structure by registering for this callback.

It's called when a client presses "Back" in the zone group's root menu.
Added logging of creating and deleting zones