Allow TileSets to support Normal Maps

So, as softwares like [SpriteLamp](http://www.snakehillgames.com/spritelamp/) are becoming popular, it seems that some people want/are using Normal Maps for Pixel Art. Some softwares, like [AutoTileGen](http://autotilegen.com/), already generates a Color Map and a Normal Map for tilesets.

So I believe Tiled could support such feature by enabling TileSets to reference a Color Map and a Normal Map.
Visualization inside Tiled would be awesome, but costly, so just letting TileSets reference two textures would be okay.

(I did not buy nor am filiated to those softwares in any way)

Start from the issue's request for TileSets to reference both a Color Map and a Normal Map. Determine how TileSets currently reference textures and define the storage needed for both maps; completion means TileSets can reference the two textures, while in-editor visualization remains optional.

Written by the indexing model from the issue text.