WorkspaceSwitcherPopup: destroy WorkspaceThumbnail objects

WorkspaceThumbnail objects must be explicitly destroy()ed, otherwise
they will not be garbage collected. Furthermore, each such object
subscribes to the "window-{left,entered}-monitor" signal of the
particular MetaDisplay. Thus leaking these thumbnails causes those
signals to have a very large number of subscribers after the popup
has been shown a sufficient number of times. This shows up in profiling,
and also causes stuttering when a window is moved between monitors or created.

For example, with 4*4=16 workspaces on 3 monitors, every time the popup
is shown 48 new subscribers are added to both signals. After a couple
days of uptime, there may be thousands.

Fix that by destroying the WorkspaceThumbnail objects in `_onDestroy()`.

wsmatrix@martin.zurowietz.de/workspacePopup/workspaceSwitcherPopup.js

@@ -195,6 +195,9 @@ class WorkspaceSwitcherPopup extends SwitcherPopup {
         while (modals.length > 0) {
             modals.pop().destroy();
         }
+
+        this._items.forEach((x) => x.destroy());
+        this._items = [];
     }
 
     vfunc_allocate(box) {