Add brushes and textures from external repositories

[BlackYps]

By defining git submodules we can add custom brushes and textures made by other people to the editor.

https://github.com/TrafficConeOfSchooling/IncoherentDimension
https://github.com/CaptainKlutz/TerrainBrushes
https://github.com/sting-2/wasteland
https://github.com/sting-2/new-realms