EPIC: define robust default directory paths for bk_client (and add-ons)

[agajdosi]

Paths used by bk_client and add-ons are based on historical evolution and sometimes are not optimal. We should check and redefine proper and robust defaults for them.

1. each OS expects binaries at some location, use those
2. find a location for bk_client store Settings store should not depend on the location of the binary #33

• global_dir (where Blender add-on saves assets) should be independent of default location for bk_client binaries (there is a use case where enterprise users want their global_dir to be set to Windows network disk, so the whole office shares the pool of downloaded assets, but Windows are extra unhappy to start a binary from network disk)
• INVESTIGATE: Is there better default global directory on Windows? Blendkit#740
• Add a fallback directory in tmp or somewhere, for situations when permissions to ~/blenderkit_data are not set Blendkit#1021