QmlProfiler: Restrict rendering of timeline to visible area

The timeline can cover a large vertical space. The introduction of
the Flickable element to navigate that space has reverted the effect
of commit 5eb057c7. This change restores the performance
improvements and avoids overflows in the underlying buffers.

Change-Id: I86ddd66652ee0a26c81619682a883622072b0f87
Reviewed-by: Kai Koehne <kai.koehne@digia.com>