#2174 One cache folder in FG_HOME or/and use user wide cache folder

[Slawek Mikula]

After some time flightgear user folder is filled with the cache subfolders. It could be reasonable to have one cache 'master' folder for all 'cache' or 'ephemeral' data. If on linux platform it could be even more reasonable to use user wide cache folder e.g. ~/.cache. Right now (as for my test home folder I have: Aircraft/ThumbnailCache, cache/, PreviewsCache/, TextureCache/* and /Terrasync (but it is rather not ephemeral)

It would be handy because:

• backup solution can easily be configured with only one cache folder to skip
• cache folder can be moved to other disk/location

[James Turner]

Yeah, we should also move this local storage on Windows indeed.

[James Turner]

Problem si everything is relative to FG_HOME, we'd need to add an FG_CACHE and then move some stuff over.