Tracing: Fix flame graph combobox

By using another style.

Also icrease the combobox width from the implicit 120px to 260px,
because the new style needs more space, and would cut off text. When we
drop Qt 5 support, we can use implicitContentWidthPolicy for that.

Fixes: QTCREATORBUG-27041
Change-Id: I438f64083c10f192125e0414640ad0b7eef2970c
Reviewed-by: Robert Löhning <robert.loehning@qt.io>

src/libs/tracing/qml/FlameGraphView.qml

@@ -28,6 +28,7 @@ import QtCreator.Tracing 1.0
 import QtQml 2.2
 import QtQuick 2.9
 import QtQuick.Controls 2.3
+import QtQuick.Controls.Material 2.3
 
 ScrollView {
     id: root
@@ -315,6 +316,7 @@ ScrollView {
 
         ComboBox {
             id: modesMenu
+            width: 260 // TODO: Use implicitContentWidthPolicy (Qt6-only)
             x: flickable.width - width
             y: flickable.contentY
             model: root.modes.map(function(role) { return root.trRoleNames[role] });