Bookshelf v2.2.1

Patch release for a crash in v2.2.0.

Fix

• Changing a book's rating from the per-book menu (long-press a book → Rating button → pick stars) crashed KOReader. The dialog hadn't seen its rating-button widget yet when its close callback was compiled, so picking a star fell into an undefined reference. Anyone hitting this had to relaunch KOReader after every rating change. Now resolves cleanly.

If you installed v2.2.0, please upgrade.

All other v2.2.0 changes still apply — see the v2.2.0 notes for the full feature list.