Allow WindowThumbnailItem work also with override redirect windows

WindowThumbnailItem doesn't depend on features that are available only
with managed window.

By making WindowThumbnailItem work with all kinds of windows, its code
is easier to work with and refactor.

src/scripting/thumbnailitem.cpp

@@ -284,7 +284,7 @@ void WindowThumbnailItem::setWId(const QUuid &wId)
     }
     m_wId = wId;
     if (!m_wId.isNull()) {
-        setClient(workspace()->findAbstractClient(wId));
+        setClient(workspace()->findToplevel(wId));
     } else if (m_client) {
         m_client = nullptr;
         updateImplicitSize();