util: Remove existing transition before adding new one

Before starting an opacity animation, ensure that there isn't one
already scheduled.

This patch fixes at least two cases of actors not showing-up:

1. When right-clicking on an item in collection view: overlay bar
doesn't become visible.
2. When launching Boxes with installer media argument: you end up in the
wizard but with main wizard area invisible.

https://bugzilla.gnome.org/show_bug.cgi?id=701635