gobject: Fix crash when disposing a GtkAccelGroup

This is a partial revert of commit fa8c7c0d using the approach suggested (and tested) by Kjell Ahlstedt.

It is intended to be temporary pending a proper dig into what’s causing the regression, just so we can get the 2.73.1 release out.

Signed-off-by: Philip Withnall pwithnall@endlessos.org

Helps: #2672 (closed)