Improved grid-layout
- The cover size pref. setting is now also applied to the grid-layout.
  An XL size was added which in portrait view will display 1 cover/book the full-width of the
  screen, and in landscape view 2 covers/books.
- Bringing up the book-context-menu can now be configured to use a long-click on the cover
  instead of the 3dot button.

FIXES:
- books without a cover are now displayed properly in list-mode.