Ok, setting more than >1 shortcuts (accels) to an action just got complicated with GTK+.
Finally looking at the code of our rival (to not name it =)), where it actually works with the default shortcut Alt+Left/Right through a GtkActionGroup (just like it's supposed to be done with GTK+), it furtermore listens to key-press events on the window itself. Should it be done the same? There is really no good way to handle multiple shortcuts for a unique action.
See patch attached, the code has an #ifdef HAVE_X11_XF86KEYSYM_H, and uses the XF86XK_Back/Forward keysyms. If such a key is pressed, it grabs the pointer on the action-group (which is available from the window class), get the action and activate it (if sensitive).
Multimedia keys work fine with menu accelerators. Problem is that a patch like this (hardcoding) can possibly lead to conflicts with daemons like music players, xfsettingsd etc.
I'd like to see this feature. It would unify the way (web|file) browsing works. Btw. and just in case it got confused: there are different keysyms for browsers (XF86Back, XF86Forward) and multimedia (XF86AudioPrev, XF86AudioNext).
I don't have a keyboard with multimedia keys, cumbersome to mess with xmodmap (and I am busy with other issues) ... though if you can, a merge request would be very welcome! (You might want to check it that 11 year old patch still applies, though I suppose not)