Decal editing

[gajop]

Add decal objects on map and do stuff with them

[gajop]

Will probably want to do this with high level of engine support, so delayed!

[gajop]

On second thought with the stuff done during LD35, it shouldn't be too hard to do it in 0.5

[gajop]

Moving this to Beta release again...
There is an existing (undocumented at the time of this writing) Lua interface for the GL4 decal implementation in the engine, but it seems incomplete, so this issue would probably do best to wait until after.