The SystenChildWindow in question is owned by ViewMediaShape
Fixed ViewMediaShape to dispose the vcl objects it creates correctly.
Also fix another crash in MediaWindowImpl child window correctly disposing its children.

Change-Id: If4aebcb6e5824266c154416f7246d73c6cb3509c
Reviewed-on: https://gerrit.libreoffice.org/16230Reviewed-by: Michael Meeks <michael.meeks@collabora.com>
Tested-by: Michael Meeks <michael.meeks@collabora.com>