meta_parse_accelerator() considers 0 length accelerator strings as
valid, meaning that the keybinding should be disabled. Unfortunately,
it doesn't initialize the MetaKeyCombo so if the caller doesn't
initialize it either, we end up using random values and possibly
grabbing random keys.
https://bugzilla.gnome.org/show_bug.cgi?id=766270
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
