QmlProfiler: Make timeline renderer vertically flickable

Introduce a second flickable element to encapsulate the timeline as
well as the labels and move SelectionRangeDetails and SelectionRange
out of that to make them float on top.

Task-number: QTCREATORBUG-10420

Change-Id: Ib4af7dc4b0155b9371c582938f1b4abb3e40cf06
Reviewed-by: Ulf Hermann <ulf.hermann@digia.com>
Reviewed-by: Kai Koehne <kai.koehne@digia.com>