Currently a transition preview continues playing until it either
completes, a different transition is selected (in which case the
new preview starts) or the user clicks on the preview. However
when "No transition" is selected whilst a preview is still playing
then the prveview continues to play.

This will also be useful for fdo#36946 which would introduce
categories into the list which themselves aren't valid transitions,
therefore we would want to stop any previews on progress when selecting
a non-transition in the list.

Change-Id: I708fcc017a94173c91cb45bef18255f97debfd57