Fix errors changing all-day event to timed event: Bug #731808

is_all_day is now a read-only property with result generated by
examining other properties, which is a better way to go (although
it does mean losing notifications when it changes; may need to
install a signal at some point).  Also, inspection of library code
indicates libical doesn't consistently replace properties and may
simpy add new ones, even if spec requires one and only one.  Going
forward, need to enforce this manually.
Miscellaneous fixes