The gsettings fallback for the bogus xkb:us::eng default only fired on
GNOME/Wayland; on GNOME/X11 the default is accepted and written back
into the X keymap, permanently switching the user off their real
layout (Hungarian, Brazilian ABNT2, etc).
_get_gnome_current_source() also had no fallback when mru-sources is
empty (a fresh session with a single input source never populates the
MRU list), so even with the guard fixed the recovery path still
returned None. It now falls back to the static sources list.
Fixes#699
