Styles: improve SH_Header_ArrowAlignment handling

The docs were wrong, and the returned values from some styles
did not match reality, so fix that. Again, this style hint was
not used at all within QWidgets...

Task-number: QTBUG-629
Change-Id: Ie6ff80fd09bc3292ba3d787ccca4d6f4c0056e89
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@theqtcompany.com>
Reviewed-by: Thorbjørn Lund Martsum <tmartsum@gmail.com>
QHeaderView: take the sort indicator into account when eliding text

By adding PM_HeaderMarkSize to the margins if the section is showing
a sort indicator. Turns out that that particular enum was actually
unused in QtWidgets (!), so tune the value to match reality.

Task-number: QTBUG-629
Change-Id: I8bc70451656d634a064c8b5014e449977c55aa9d
Reviewed-by: Cristian Oneț <onet.cristian@gmail.com>
Reviewed-by: Thorbjørn Lund Martsum <tmartsum@gmail.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@theqtcompany.com>