QmlProfiler: Provide a horizontal scroll bar for the timeline

This requires the consolidation of the nested Flickable elements into
one, which is probably a good idea anyway. The horizontal scroll bar is
important because people might not understand that they can use the
overview for scrolling.

Change-Id: Ie1555265fc3edafaf6e6e4f34d77b0d034d45639
Reviewed-by: Kai Koehne <kai.koehne@digia.com>