keybindings: Don't use the keysym to match keybindings

We don't want to match the keysym so that e.g. an accelerator
specified as "<Super>a" works if the current keymap has a keysym other
than 'a' for that keycode which means that the accelerator would
become inaccessible in a non-latin keymap.

This is inconvenient for users that often switch keyboard layouts, or
even have different layouts in different windows, since they expect
system-level keybindings to not be affected by the current layout.

https://bugzilla.gnome.org/show_bug.cgi?id=678001