Showing underline in accelerators by default

[Miguel Guedes <miguel a guedes gmail com>]

On XFCE (and GTK+2) all the themes by default hide the keyboard 
accelerators of buttons, menu items, etc, and they only become visible 
once ALT is depressed. I find this irritating as I am a heavy keyboard 
and find that having to depress ALT to view available shortcuts cuts the 
flow of thoughts/actions. 

I've tried editing the theme I use and set gtk-auto-mnemonics to 0 but 
then the accelerators aren't shown at all, ALT depressed or not.

Is there a different setting I should be changing instead?