Tracing: Restrict category tooltip to label text

We don't want the tooltip to show up on the disabled expand button.

Change-Id: I0ac6f185a6d6a5e01d07e1dcdacba736816f151e
Reviewed-by: Tim Henning <tim.henning@kdab.com>
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>

src/libs/tracing/qml/CategoryLabel.qml

@@ -56,7 +56,7 @@ Item {
 
     Button {
         // dummy button to display a tooltip
-        anchors.fill: parent
+        anchors.fill: txt
         tooltip: labelContainer.text
         style: ButtonStyle {
             background: Item {}