gtkimmulticontext: Handle switches between displays

gtkimmulticontext: Handle switches between displays

Currently the GtkIMMultiContext may stick to a delegate GtkIMContext
that no longer applies after the multicontext is dissociated from
any widget.

Handle set_client_widget() so that it can handle changes between
widgets from 2 different display, but also so the delegate is made
NULL whenever the context has a NULL widget.

Doing so, any new client widget results in a new delegate IM context
lookup from the right GdkDisplay and GtkSettings, which avoids any
mix up.

Fixes: https://gitlab.gnome.org/GNOME/gtk/-/issues/4805