gutterrendererchunk: Update to new popup API and fix positioning

Author: kaiw

meld/gutterrendererchunk.py

@@ -205,9 +205,13 @@ def do_activate(self, start, area, event):
             self.filediff.delete_chunk(self.from_pane, chunk)
         elif action == MODE_INSERT:
             copy_menu = self._make_copy_menu(chunk)
-            # TODO: Need a custom GtkMenuPositionFunc to position this next to
-            # the clicked gutter, not where the cursor is
-            copy_menu.popup(None, None, None, None, 0, event.time)
+            copy_menu.popup_at_rect(
+                self.get_view().get_window(self.get_window_type()),
+                area,
+                Gdk.Gravity.SOUTH_WEST,
+                Gdk.Gravity.NORTH_WEST,
+                None,
+            )
         elif action == MODE_REPLACE:
             self.filediff.replace_chunk(self.from_pane, self.to_pane, chunk)