Blender Git Loki

Cycles: Use XDG's .cache folder for cached kernels

Basically just moves cached kernels from ~/.config/blender/BLENDER_VERSION to
~/.cache/cycles/kernels. This has following benefits:

- Follows XDG specification more closely,
not as if it's totally crucial or measurable by users, but still nice.

- Prevents unexpected sizes of config folder, makes disk space used in more
predictable for users way.

- Allows to share kernels across multiple Blender versions,
which makes it easier debugging at the times close to release.

- "Copy Previous Settings" operator will no longer be copying possibly
gigabytes of cached kernels, which used to lead to really nast disk usage
and annoying delays of copying settings.

- In the future we can have some smart logic to clear old unused cached
kernels.

Currently only done for Linux and OSX. Windows still follows old "cache"
folder logic, but it's not really important for now because we don't
support kernel compilation on this platform yet.

Reviewers: dingto, juicyfruit, brecht

Reviewed By: brecht

Differential Revision: https://developer.blender.org/D2197