Timeline: Don't anchor delegate items

Apparently the parent is not always defined and we get warnings about
that.

Change-Id: I574d68c2668fef53af9422049143a0decf05286f
Reviewed-by: Joerg Bornemann <joerg.bornemann@theqtcompany.com>

src/libs/timeline/qml/TimelineLabels.qml

@@ -78,10 +78,9 @@ Flickable {
             delegate: Rectangle {
                 color: categories.color
 
-                anchors.left: parent.left
-                anchors.right: parent.right
                 property int visualIndex: DelegateModel.itemsIndex
                 height: label.visible ? label.height : 0
+                width: categories.width
 
                 CategoryLabel {
                     id: label