window: pop up recurrence-dialog right after edit-dialog

The recurrence-dialog opens only if the event to be modified
is a recurring event and only if the user doesn't cancel
the modification in the edit-dialog. If the user presses
'Cancel' in the recurrence-dialog after making modifications
in the edit-dialog, then no changes are applied.

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