Add a thing scale slider
This scales things in both classic modes and 3d mode. This is on top of the x/y scale of the thing.
As a bonus, things are now scaled by x scale in classic modes.
Added to the Interface tab of Preferences because we have no SOC parser right now. It's kind of squeezed in there but there's nothing I can do about that -- the UI code for Doom Builder sucks big balls!