workspace-indicator: Initialize preview visibility

Otherwise we can end up showing windows 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/workspace-indicator/extension.js

@@ -49,6 +49,7 @@ 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);