Improve efficiency of qmlEngine()
Improve efficiency of qmlEngine()

Also document qmlEngine() and qmlContext()

Change-Id: I83eb092490160eee72d87473befdcea1492ded13
Reviewed-on: http://codereview.qt-project.org/4491
Reviewed-by: Qt Sanity Bot <5581206bb7e0307f0d99eb71898ae6b694149ca5@ovi.com>
Reviewed-by: Aaron Kennedy <087c9f3fe24ea980d31d6a327c000f1b04b5a585@nokia.com>

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