TE: Add GIMP xcf support

[Castigavi]

Currently we support the generic formats like tga, but we also have full support of PSD file type. Considering Photoshop is a closed source application it feels only natural that GIMP is included too, especially since TR community is actually using this app to create textures (eg Dejan, but also TRF even has tutorials for GIMP).

Because of that I propose adding support to GIMP's project format - XCF. That way we would support one format for well known closed source app (Photoshop) and open source one (GIMP). This is already a thing with 3d apps, since we support both Blender and 3ds max.