removed: BackGroundInfo Loader multithreading (no longer needed)
changed: Split background info loading into a 2 stages

This allows us to first perform loading of cached (fast) stuff and after that load the lookup (slow) stuff (partially fixes #14071)
changed: Renamed m_database to m_video/m_musicDatabase for clarity
changed: Use a member variable for the texture database

This speeds up thumb loading considerably. This also gets rid of the Initialize/Deinitialize ThumbLoader functions
fixed: Music thumb loading for files view was slow

This was caused by the fact that we never checked for cached images for music items
fixed: Thumb/flag extractions were never stored when used outside the bg infoloader

Also factored out some (obsolote?) code as a bonus
Merge pull request #2890 from arnova/thumb_loader_refactor

Thumb loader refactor