QtQuick.Controls TextField: add remove() function

Autotest is included.

[ChangeLog][QtQuick.Controls] TextField: add remove() function

Task-number: QTBUG-40735
Change-Id: If73ec284da6d0306a7f01d001f2b762a9369f61d
Reviewed-by: Mitch Curtis <mitch.curtis@digia.com>
Reviewed-by: Shawn Rutledge <shawn.rutledge@digia.com>