We only want to subscribe to the mouse pointer events of the document
window, not anything else.

Change-Id: Ie21db52b2dd0d4f55289896caba5174380316e65
Reviewed-on: https://gerrit.libreoffice.org/50412Reviewed-by: Jan Holesovsky <kendy@collabora.com>
Tested-by: Jan Holesovsky <kendy@collabora.com>