280: Remove Composite Textures feature

[tngreene]

Composite Textures should be removed because

• Substance Painter and EEVEE make it obsolete, the feature can't match

• It isn't useful for PBR (I think)

• I haven't seen much reports of people using it (any well used feature usually has some bugs or feature requests)

• You can accomplish the same thing quickly in Photoshop or GIMP, the process doesn't use any special Blender properties aside from auto detect figuring out what textures to use (which we've removed anyway)

• While cleaning unit tests, I've disabled basically everything that uses the feature

• See if anybody responds to this bug request saying its an essential part of their workflow

• Remove code from xplane_header.py

• Delete xplane_image_composer.py (a file so little used, I had completely forgotten of its existence - and its in the top directory)

[tngreene]

If, after the next RC, I make a poll of developers and no one says they'll miss this - we'll remove it

[tngreene]

d84881d nearly solves this. We need to replace the build number in updater to be the actual build number and such for when we really get close to beta.3 when this is planned for

• Replace build number before actual release