Added seek() function to QML Video item and updated documentation.
Added seek() function to QML Video item and updated documentation.

The Video item inline example was wrong, changing read-only properties.

Task-number: QTBUG-28741

Change-Id: Ia1f6c1110a301a96743e7ebfec51731fcaf6a862
Reviewed-by: Martin Smith <a8cc6484db2755056ccf1c457da8ca771f895dea@digia.com>
Reviewed-by: Dmytro Poplavskiy <72e04cfb587e72184d425a79cb5cea0021f0b982@gmail.com>

nezticle/qtmultimedia,sergiusens/qtmultimedia,sergiusens/qtmultimedia,jfaust/qtmultimedia,webOS-ports/qtmultimedia,nezticle/qtmultimedia,jfaust/qtmultimedia,webOS-ports/qtmultimedia,sergiusens/qtmultimedia,jfaust/qtmultimedia,webOS-ports/qtmultimedia,mer-qt/qtmultimedia,mer-qt/qtmultimedia,nezticle/qtmultimedia,nezticle/qtmultimedia,mer-qt/qtmultimedia,jfaust/qtmultimedia,sergiusens/qtmultimedia,mer-qt/qtmultimedia,webOS-ports/qtmultimedia,nezticle/qtmultimedia,jfaust/qtmultimedia,webOS-ports/qtmultimedia,sergiusens/qtmultimedia,mer-qt/qtmultimedia