window.ui: Don't destroy GcalSourceDialog on close.

Since the change in d5cd6c7fc9 closing
the source dialog actually destroyed it and could not be used again.

Use gtk_widget_hide_on_delete to handle the delete-event.

https://bugzilla.gnome.org/show_bug.cgi?id=764280

data/ui/window.ui

@@ -343,6 +343,7 @@
   <object class="GcalSourceDialog" id="source_dialog">
     <property name="visible">False</property>
     <property name="transient_for">GcalWindow</property>
+    <signal name="delete-event" handler="gtk_widget_hide_on_delete" object="GcalWindow" swapped="no" />
   </object>
   <object class="GcalEditDialog" id="edit_dialog">
     <property name="visible">False</property>