Caching - Delete old, long unused figures from cache, when configured to do so

[kutis96]

Implementation ideas

• Cache file deletion may be explicitly disabled
• If automated cache deletion has been enabled:
  • Delete cached items if they weren't used during the last run, and their TTL (time-to-live) hasn't ran out
    • TTL - users may revert back to older versions while they're editing, but it seems unlikely such a thing would happen after some period of development, when the user would focus on other parts of the document. Old development versions of these files may be removed again.
    • Update file modified/accessed attributes whenever the cached files are actually used, to actually track TTL
      • This could be also done in a separate file storing such attributes for all cached, but why do that when one can abuse the FS this way :')

Configurability:

• Whether to actually enable caching at all
• Default cache directory
• Whether to ever delete cache items for unused files
• What maximum TTL should unused cached items have