ui: add helpers for virtio-multitouch events

Add helpers for generating Multi-touch events from the UI backends that
can be sent to the guest through a virtio-multitouch device.

Signed-off-by: Sergio Lopez <slp@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20230526112925.38794-6-slp@redhat.com>