Add optional width and height arguments to mapToItem()/mapFromItem()

When specifying the width and height from QML, mapToItem()/mapFromItem()
will use QQuickItem::mapRectToItem (and mapRectFromItem).

Task-number: QTBUG-25755
Change-Id: Iae87671913623969d7428dd008afaa6faf3faf33
Reviewed-by: Martin Jones <martin.jones@nokia.com>