meta_accelerator_parse(): handle keysyms without the XF86 prefix

The GDK and hence GNOME standard is that keys that begin with XF86 according to
libxkbcommon not prefixed with XF86, though gdk_keyval_from_name() strips XF86
if provided. If libxkbcommon doesn't recognize the accelerator name without
XF86, try again adding XF86 to the start.

This restores compatibility with gnome-settings-daemon, schemas, and existing
user configuration.

https://bugzilla.gnome.org/show_bug.cgi?id=727993