Bug Fixes:
I#68 - WebDAV registry backend Refresh can remove existing sources (Milan Crha)
M!7 - [CalDAV] Perform free-busy-query on the correct principal (Jamie McClymont)
M!8 - [CalDAV] Invert success value of schedule outbox free_busy (Jamie McClymont)
Miscellaneous:
Correct developer documentation of some EBook asynchronous API (Milan Crha)
Crash on meeting cancellation receive in local calendar (Milan Crha)
Fix few memory leaks in calendar backends (Milan Crha)
