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

js/ui/environment.js

@@ -97,6 +97,8 @@ function _getPropertyTarget(actor, propName) {
         return [actor.get_action(name), prop];
     case '@constraints':
         return [actor.get_constraint(name), prop];
+    case '@content':
+        return [actor.content, name];
     case '@effects':
         return [actor.get_effect(name), prop];
     }