environment: Handle @content for property transitions

When using Actor.ease_property if the property starts with '@' and the duration of the transition is zero (which may happen if the actor is not mapped even if a non-zero duration was passed to ease_property), the impl will try getting the actual target object where the property should be set.

This works fine for most cases but it currently throws an error when passing '@content.*' properties. Fix this by handling '@content' as a property of actor.content (used by MetaBackgroundActor when showing the overview).