[gnome-shell] overview: Add a 'closing' getter

[Marge Bot <marge-bot src gnome org>]

commit 74d0b066e9b6d1e434e1bb5f31b29440b5a24b69
Author: Daniel van Vugt <daniel van vugt canonical com>
Date:   Wed Sep 16 14:52:19 2020 +0800

    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 | 4 ++++
 1 file changed, 4 insertions(+)
---
diff --git a/js/ui/overview.js b/js/ui/overview.js
index c71b113898..12b9669272 100644
--- a/js/ui/overview.js
+++ b/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;