Allow manage Height on polyzone creation tool

[davedevils]

I edit very often PolyZone height manually, so i just edited for make it work like BoxZone and manage height directly on creation tool without lost time by manually find height, that allow to simply manage ingame.

[mkafrin]

Thanks for the PR. Will review when I get a chance. One thing I see on first quick look is I think I'll end up wanting the height defaults to match BoxZones (z - 1, z + 3), but I'll make a final decision when I review fully.