window-list: Initialize preview visibility

Otherwise we can end up showing window previews in the workspace
thumbnail that shouldn't be visible, if none of the properties we
track for updates changes.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell-extensions/-/merge_requests/280>

extensions/window-list/workspaceIndicator.js

@@ -43,6 +43,7 @@ class WindowPreview extends St.Button {
             },
             'notify::minimized', this._updateVisible.bind(this),
             this);
+        this._updateVisible();
 
         global.display.connectObject('notify::focus-window',
             this._onFocusChanged.bind(this), this);