games/SLADE: Import version 3.1.12a

SLADE3 is a modern editor for Doom-engine based games and source ports.
It has the ability to view, modify, and write many different game-specific
formats, and even convert between some of them, or from/to other generic
formats such as PNG.

SLADE3 can be considered a successor to both SLumpEd and SLADE - it combines
the features of both, to create an all-in-one editor. Why does it keep the
name of what was previously just a map editor? Because it fits :)