SortFilterProxyModel for QML (make QSortFilterProxyModel available)

Make QSortFilterProxyModel available in Qt Quick.

There is someone interested in contributing their work it seems - Grecko's SortFilterProxyModel https://github.com/oKcerG/SortFilterProxyModel; getting it into Qt means API cleanup, but the proof of concept is there.

• support column reordering (including header data)
• perhaps support transforming the model horizontal vs vertical (like ListView)
• perhaps support changing layout right-to-left instead of left-to-right (like ListView)
• sorting
• filtering
• support for querying row heights and column widths from QQuickTableView
• should work with QQuickHeaderView, so that if you rearrange columns there, the model should stay in sync
• Be able to override rowCount and columnCount (or rowStart, rowEnd, columnStart, columnEnd), in case you only want to show a section of the model
• support hiding individual columns