Blender Git Loki

Fix broken logic in new FileBrowser caching of entries.

We were still using main borwser's FileDirEntryArr to own memory of entries currently in cache.

That was OK for common caching operation, but if was not released when clearing cache!
Nothing dramatic, since that was cleared too, but less often, so e.g. by changing sorting
options you could pile up more and more no-more-used entries there...

Anyway, there is no reason to do that, much more sound and clean to keep everything
cache-related in dedicated FileDirEntryCache struct.

This means main borwser's FileDirEntryArr is now always expected to not store any actual entry!

Note: this can probably be cleaned up a bit more, but would wait after 2.76 for this!