Added multiple engine support for any game configuration.

Floor and ceiling textures are now moved more predictably by arrow keys in GZDoom Visual mode.
Walls texture coordinates are always rounded when moved by arrow keys in Visual modes.
Linedef info panel: relative UDMF light values are now shown like this: 16 (128), which means "UDMF light value" ("total surface brightness (UDMF light value + sector brightness)")
Player is now spawned at camera height when testing from current location in Visual modes.
Focus is now properly restored after testing from current location in Visual modes.
Updated Heretic_sectors.cfg and Heretic_things.cfg as described here: http://www.doombuilder.com/forums/viewtopic.php?f=11&t=357
Changed sprites of artifacts in Hexen_things.cfg to proper ones.
Renamed Skulltag configs to Zandronum