workspace-indicator: Update visibility on window-type changes

The visibility depends on the type, so we should listen for
changes (even if the property is unlikely to change).

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell-extensions/-/merge_requests/292>
(cherry picked from commit 30a15387b5)

extensions/workspace-indicator/extension.js

@@ -45,6 +45,7 @@ class WindowPreview extends St.Button {
             'size-changed', () => this._checkRelayout(),
             'position-changed', () => this._checkRelayout(),
             'notify::minimized', this._updateVisible.bind(this),
+            'notify::window-type', this._updateVisible.bind(this),
             this);
         this._updateVisible();