graphicsview: use std::sort instead of qSort

graphicsview: use std::sort instead of qSort

In almost all cases, std::sort is wildly faster than qSort - but especially in
the case where the input data is already sorted. in some stress tests which ran
through the index with a lot of items, this commit provides huge speedup (684ms
down to 10ms for painting 15001 empty items on the provided benchmark), for me.

Task-number: QTBUG-11022
Change-Id: I5551f8e320c33ba13d464bf22047a665c81f3b74
Reviewed-by: Peter Kümmel <syntheticpp@gmx.net>
Reviewed-by: João Abecasis <joao.abecasis@nokia.com>