re-add used signal.

data/gui/input_dialog.ui

@@ -7,6 +7,7 @@
     <property name="type_hint">dialog</property>
     <property name="has_separator">False</property>
     <signal name="destroy" handler="on_input_dialog_destroy"/>
+    <signal name="delete_event" handler="on_input_dialog_delete_event"/>
     <child internal-child="vbox">
       <object class="GtkVBox" id="dialog-vbox10">
         <property name="visible">True</property>

src/dialogs.py

@@ -1835,6 +1835,12 @@ class InputDialog(CommonInputDialog):
         if input_str:
             self.set_entry(input_str)
 
+    def on_input_dialog_delete_event(self, widget, event):
+        '''
+        may be implemented by subclasses
+        '''
+        pass
+
     def set_entry(self, value):
         self.input_entry.set_text(value)
         self.input_entry.select_region(0, -1) # select all