xsettings: Always fallback to ibus

Traditionally, we've used ibus strictly when necessary (i.e. there
was an IM configured that required IBus to be enabled). A bit more
recently, we added OSK checks so that environments with an OSK
would also have IBus enabled so that GNOME Shell could track focus
positions without pulling a11y.

Nowadays, we simply want ibus always enabled, even if the underlying
keymap wouldn't seemingly use its features. We anyways want it for
the handling of emojis and dead keys.

Always enable IBus for X11 clients, unless there is another IM
configured in the global GSetting.

Closes: https://gitlab.gnome.org/GNOME/gnome-settings-daemon/-/issues/705