event: add recurrence property

Earlier, an event had no recurrence property. Hence, the event's
recurrence pattern etc. could not be analysed, and as a result
could not be changed by the user. gnome-calendar just displayed
all the recurrences of a recurring event but had no 'readable'
knowledge regarding the characteristics of those recurrences.

This patch adds the recurrence property, which is basically
a GcalRecurrence structure linked to the event.

https://bugzilla.gnome.org/show_bug.cgi?id=782755