QmlProfiler: Use anchors to determine the size of the timebar background

For some reason, when the timeline width is 0, the background width can
get negative, if using bindings; it works fine when using anchors.

Change-Id: I5eb76f83b9c41c1a71a135082770079cedc7f16c
Reviewed-by: Kai Koehne <kai.koehne@digia.com>