Import Wavefront .obj as Terrain mode now works in all map formats.

Import Wavefront .obj as Terrain mode: floor height of each created sector is now set to the average height of an .obj polygon it was created from.
Import Wavefront .obj as Terrain Settings window: added "Use slopes" checkbox. When enabled, the mode will create slopes using vertex height offsets (UDMF) or Floor Vertex Height things (1504).
Fixed, Visual Mode: things were rendered at wrong height after using "Lower/Raise Height" actions in sectors with sloped floor (or ceiling for ceiling-aligned things) 
Fixed: info panel was not updated after switching to another Edit Mode.