QmlProfiler: Reorganize timeline into rows

This potentially allows us to use different views for different
categories and reduces the reliance on TimelineModelAggregator.

Change-Id: I486481599d1517abc0087c565358f27405e4108b
Reviewed-by: Kai Koehne <kai.koehne@theqtcompany.com>
QmlProfiler: Remove most of timelinemodelaggregator

Change-Id: If56940133b54905700fd51feac61d097677002c2
Reviewed-by: Kai Koehne <kai.koehne@theqtcompany.com>