Accelerator in global menu doesn't update when dynamically changed

How to reproduce:

While using the attached test application, the menu first shows "Quit Ctrl+Q". Click on the "Change menu" button.

Result: As expected, the menu entry now reads "Test", and the actual accelerator has changed to "Ctrl+T". However, the menu still shows "Ctrl+Q" as accelerator, while one would expect it being updated too.

(I tested the code with the recent Ubuntu Precise Pangolin Beta 1, and the bug is still there).

Note: while I'd be interested in a workaround, e.g. another way of dynamically changing the accelerator, I think the attached code is a perfectly valid use of the GTK+ lib, since it works with both GNOME Shell and GNOME "Classic". Unity should support it too.