overview: Add a 'closing' getter

Which better communicates what we are checking for, and is a little more
elegant than repeatedly writing:
```
Main.overview.animationInProgress && Main.overview.visibleTarget == false
```

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1440>

js/ui/overview.js

@@ -134,6 +134,10 @@ var Overview = class {
         return this._visibleTarget;
     }
 
+    get closing() {
+        return this._animationInProgress && !this._visibleTarget;
+    }
+
     _createOverview() {
         if (this._overview)
             return;