filediff: Fix change deletion shortcut for non-keypad deletes

This was broken by 30bd323, because you can't actually expect multiple
accelerators to work.

The change here is kind of unpleasant. Really, it would be nice if
this was a generic helper that ran on focus, probably as part of MeldDoc
handling... but that's very overkill for this one accelerator.

Author: kaiw

data/ui/filediff.ui

@@ -132,7 +132,6 @@
         <signal name="activate" handler="delete_change" swapped="no"/>
       </object>
       <accelerator key="Delete" modifiers="GDK_MOD1_MASK"/>
-      <accelerator key="KP_Delete" modifiers="GDK_MOD1_MASK"/>
     </child>
     <child>
       <object class="GtkAction" id="MergeFromLeft">

meld/filediff.py

@@ -191,6 +191,11 @@ def __init__(self, num_panes):
         self.actiongroup = self.FilediffActions
         self.actiongroup.set_translation_domain("meld")
 
+        # Alternate keybindings for a few commands.
+        self.extra_accels = (
+            ("<Alt>KP_Delete", self.delete_change),
+        )
+
         self.findbar = FindBar(self.grid)
         self.grid.attach(self.findbar.widget, 1, 2, 5, 1)
 
@@ -279,6 +284,22 @@ def go_to_line(widget, line, pane):
 
         self.connect("notify::ignore-blank-lines", self.refresh_comparison)
 
+    def on_container_switch_in_event(self, ui):
+        MeldDoc.on_container_switch_in_event(self, ui)
+
+        accel_group = ui.get_accel_group()
+        for accel, callback in self.extra_accels:
+            keyval, mask = Gtk.accelerator_parse(accel)
+            accel_group.connect(keyval, mask, 0, callback)
+
+    def on_container_switch_out_event(self, ui):
+        accel_group = ui.get_accel_group()
+        for accel, callback in self.extra_accels:
+            keyval, mask = Gtk.accelerator_parse(accel)
+            accel_group.disconnect_key(keyval, mask)
+
+        MeldDoc.on_container_switch_out_event(self, ui)
+
     def get_keymask(self):
         return self._keymask