Tracing: Add tooltip to category labels in analyzer timelines

This is especially useful for longer names that often get elided in the
Perf Profiler and the new Chrome Trace Format Visualizer.

It uses a dummy button to show the same tooltip style as the buttons
while not having to import QtQuick.Controls.Private.

Change-Id: I5580a9ecc1acb69b0faec00a2f35952df0e1ce92
Reviewed-by: Ulf Hermann <ulf.hermann@qt.io>

src/libs/tracing/qml/CategoryLabel.qml

@@ -54,6 +54,15 @@ Item {
 
     property bool reverseSelect: false
 
+    Button {
+        // dummy button to display a tooltip
+        anchors.fill: parent
+        tooltip: labelContainer.text
+        style: ButtonStyle {
+            background: Item {}
+        }
+    }
+
     MouseArea {
         id: dragArea
         anchors.fill: txt