Media Library: further iteration

This MR further iterates on the local media library

• ensure thumbnail and the background share the same size
  • This way, the title label for group and thumbnail will also be on the same height across cells.
• reduce corner radius on thumbnails in movie section (see screenshot below)
• collection model: observe thumbnails so they are displayed as they become available
• media category view controller: request up to 4 thumbnails per group
  • This way, newly added groups will show up to four thumbnails instead of index 0 only and then placeholders until the thumbnails are requested individually