edit-dialog: correctly assign repeat widgets' sensitivity

Earlier repeat widgets were sensitive even when
the event selected was read-only. This patch corrects
this behavior and makes the repeat widgets insensitive
when the event is read-only.

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

data/ui/edit-dialog.ui

@@ -437,6 +437,7 @@
                 <property name="margin-top">12</property>
                 <property name="hexpand">True</property>
                 <signal name="changed" handler="repeat_type_changed" object="GcalEditDialog" swapped="no"/>
+                <property name="sensitive" bind-source="GcalEditDialog" bind-property="writable" bind-flags="default" />
                 <items>
                   <item translatable="yes" id="no_repeat">No Repeat</item>
                   <item translatable="yes" id="daily">Daily</item>