TextField/Area: soft dependency to inputMethodHints

Don't use an alias to a property that might not exist (when QT_NO_IM),
but just bind to it and fallback to Qt.ImhNone if it's undefined.

Change-Id: I0e8de86acb534dfe620b829c2bff09db119ef08d
Reviewed-by: Liang Qi <liang.qi@digia.com>