Slideshow: Fix regression related to media animations

Misc effects tab page animations were lost.
Regression from
9695f38ca4ab26b40524f2eade98c45f0360131d.

Change-Id: I81f7321d7dd87f5b0f6b4acbecbd60241ab1e430
(cherry picked from commit 81e98150cc04ad764abc668dc12b91d3de90369e)
Reviewed-on: https://gerrit.libreoffice.org/9054
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>