Fixes and polish for 5.1.
- Returning to the shelf after closing a book is much quicker (#422).
- Shelves showing folders open much faster (#409).
- The hero no longer glitches when you switch to a shelf with nothing in it (#423).
- Shelf colours no longer come out inverted when night mode is switched by something other than KOReader (#426).
- The shelf boards keep their wood colour with the shelf theme pinned to Light or Dark.
- Spine mode follows KOReader's "Folders and files mixed" setting.
- "Recently added" faces out that many books across the shelf, rather than that many on every screen.
- Face out the first, or the first unread, book of a series on a shelf with no series grouping (#425).
- The label under a cover follows the Panel shading setting.
- Tidier outlines in the shelf menu, and the drilled-in view now matches the top level.