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_videoDatabase and m_musicDatabase for clarity
changed: Use a member variable for the texture database to speedup thumb loading
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