environment: Handle @content for property transitions

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).

https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1461