hmm yes, I know what is all the mnemonic all about. But I already set gtk_menu_item_new_with_mnemonic(_("Switch _Focus")); so I do not know what is the mnemonic in keybindings_set_item intended for.
I'm sorry, pressing the alt key on Ubuntu Unity has a rather odd behaviour since it launches the HUD Unity feature, so I don't use mnemonics very often.