filediff: Handle save responses better in delete events (bgo#744081)

The "you have modified files" dialog offers a "Close without saving"
option, which returns a CLOSE response code instead of our usual OK.
This was being handled correctly in most places, but not when the tab
was being closed via a delete event. This patch fixes it so that the
check_save_modified() always returns either OK or CANCEL, since that's
really the only thing we need out of it.

This commit also removes a stray delete-event handler on a widget that
should never ever be loaded.

Author: kaiw

data/ui/filediff.ui

@@ -280,7 +280,6 @@
     <property name="visible">True</property>
     <property name="can_focus">False</property>
     <property name="title" translatable="yes">window1</property>
-    <signal name="destroy" handler="on_delete_event" swapped="no"/>
     <child>
       <object class="GtkVBox" id="filediff">
         <property name="visible">True</property>

meld/filediff.py

@@ -883,6 +883,9 @@ def check_save_modified(self):
             elif response == Gtk.ResponseType.DELETE_EVENT:
                 response = Gtk.ResponseType.CANCEL
 
+        if response == Gtk.ResponseType.CLOSE:
+            response = Gtk.ResponseType.OK
+
         if response == Gtk.ResponseType.OK and self.meta:
             parent = self.meta.get('parent', None)
             saved = self.meta.get('middle_saved', False)