calendar: Permanently hide dismissed events

Currently dismissed events will simply reappear when browsing
back and forth between dates, which is clearly broken. Instead,
hide events that have been dismissed permanently. For now, we
simply store a list of ignored IDs ourselves, until we get API
in evolution-data-server to reliably store custom per-event
properties.

https://bugzilla.gnome.org/show_bug.cgi?id=744927