Additional ListView section header placement options

Additional ListView section header placement options

Add a section.labelPositioning property which can be a combination
of:

- ViewSection.InlineLabels - section labels are shown inline between
  the item delegates separating sections (default).
- ViewSection.CurrentLabelAtStart - the current section label sticks to
  the start of the view as it is moved.
- ViewSection.NextLabelAtEnd - the next section label (beyond all visible
  sections) sticks to the end of the view as it is moved.

Task-number: QTBUG-12880
Change-Id: I4601828337412bd3a83769c9b8df3f6d4d7474b8
Reviewed-on: http://codereview.qt-project.org/5192
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Bea Lam <bea.lam@nokia.com>