window: pop up recurrence-dialog only when required

Earlier, the recurrence-dialog would pop up even when user only
wanted to change the recurrence-property of an already recurring
event. This is incorrect behavior.

This patch fixes this issue by making sure that recurrence-dialog
doesn't pop up when the new recurrence-property chosen by the
user is different from what the event already has. In this case,
the user only wants to change the recurence-property of the event
and there's no use of the recurrence-dialog.

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