Contruct per-delegate QQmlContextData directly.

Contruct per-delegate QQmlContextData directly.

The QQmlContext object created per item is a weighty (QObject) wrapper
around QQmlContextData that isn't utilized except as an intermeditrary
for the QQmlComponent API.  By duplicating a little code out of
QQmlComponent we can avoid the need to ever contruct the wrapper.

Change-Id: I74558c7e746ead2c5127a8754d5f04550b8f4d10
Reviewed-by: Martin Jones <martin.jones@nokia.com>