utils: add dialog to modify recurring events

Whenever a user chooses to modify an event which is
recurring, a new dialog will open automatically when
the user clicks 'Done' or 'Delete' in edit-dialog,
after making the desired changes. This new dialog
will ask the user to choose the set of events to
which the changes will be applied. The options
include 'This Event Only', 'Subsequent Events'
and 'All Events'. The function returns true and
the changes take place only if the user chooses any
option other than 'Cancel' or <Alt+F4>.

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

src/gcal-utils.h

@@ -139,4 +139,8 @@ gboolean        should_change_date_for_scroll                   (gdouble
 
 gboolean        is_source_enabled                               (ESource               *source);
 
+gboolean        ask_recurrence_modification_type                (GtkWidget             *parent,
+                                                                 ECalObjModType        *modtype,
+                                                                 ESource                source);
+
 #endif // __GCAL_UTILS_H__