Timeline: Pass the right dragOffset to CategoryLabels

Also, make sure the category dragger only reacts to vertical mouse
movements.

Change-Id: Ic53db9030536a1f2d35ea02facb920a4d3f0a005
Task-number: QTCREATORBUG-15333
Reviewed-by: Joerg Bornemann <joerg.bornemann@theqtcompany.com>

src/libs/timeline/qml/CategoryLabel.qml

@@ -63,6 +63,7 @@ Item {
         cursorShape: dragging ? Qt.ClosedHandCursor : Qt.OpenHandCursor
         drag.minimumY: dragging ? 0 : -dragOffset // Account for parent change below
         drag.maximumY: draggerParent.height - (dragging ? 0 : dragOffset)
+        drag.axis: Drag.YAxis
     }
 
     DropArea {

src/libs/timeline/qml/TimelineLabels.qml

@@ -102,7 +102,7 @@ Flickable {
                         draggerParent: categories
                         width: 150
                         height: parent.height
-                        dragOffset: parent.y
+                        dragOffset: loader.y
 
                         onDropped: {
                             categories.moveCategories(sourceIndex, targetIndex);