Move QPainterPathPrivate to the private header files.

Move QPainterPathPrivate to the private header files.

This was made inline for the sake of performance, but has in hindsight
proven to be not really worth it. It also prevents us from aligning
the internal datastructure of QPainterPath with that of QPolygonF
and the internal QVectorPath class which would make mapping
between the two inside QPainter a lot faster.

Making all this out-of-line now as discussed in the task
https://bugreports.qt-project.org/browse/QTBUG-19998
so we can adress this in a binary compatible fashion during
Qt 5.x

Change-Id: I61058171ed31f8a845fa45517248367c85ce52cd
Reviewed-by: Lars Knoll <lars.knoll@nokia.com>
Reviewed-by: Samuel Rødal <samuel.rodal@nokia.com>