recurrence: add a function to check equality of recurrences

The gcal_recurrence_is_equal function checks if two recurrences are
equal or not. When a user is trying to modify the recurrence-rules
of an already recurring event, this function tells us if the
recurrence-dialog should pop up or not.

This patch also removes some unused code.

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

src/gcal-recurrence.h

@@ -67,6 +67,9 @@ GcalRecurrence*      gcal_recurrence_copy                        (GcalRecurrence
 
 void                 gcal_recurrence_free                        (GcalRecurrence *recur);
 
+gboolean             gcal_recurrence_is_equal                    (GcalRecurrence *recur1,
+                                                                  GcalRecurrence *recur2);
+
 GcalRecurrence*      gcal_recurrence_parse_recurrence_rules      (ECalComponent *comp);
 
 struct icalrecurrencetype*    gcal_recurrence_to_rrule           (GcalRecurrence *recur);