events: fix handling of event with no end date

Fix the sigsegv in edit-dialog and event-widgets when there's no
end-date set.
Set a guard in Utils::gcal_dup_icaltime function.
There's still some problems with the events showing the first time but
not anymore.

Bug: https://bugzilla.gnome.org/show_bug.cgi?id=736002
manager: defer adding client to the internal hash

Wait until the client is opened and connected to add it to the internal
hash, so GcalManager::get_sources work properly.