Ability to change the region selection limit to base itself on a 2D selection rather than 3D

WORLDGUARD-3473 - Reported by sanovskiy

As the title states. 30,000 block limit in 3D isn't much, but anything higher could reach the point of "too much space." The ability to set the limit as basing it on the X and Z lengths rather than all three would help.

Clone of http://youtrack.sk89q.com/issue/WORLDGUARD-2561