Allow using multi fontengine in QTextLayout with QRawFont.

Allow using multi fontengine in QTextLayout with QRawFont.

This change enables us to instantiate a QFontEngineMulti that takes
the raw font's font engine as its primary engine but can use fallback
engines based on the platform. Since this can be quite expensive, we
defer the query for fallback families' names until it's needed and
we cache the resulting multi font engine.

Change-Id: I390dbc1cb2fe61d56867f29a03f313eb3eb49dc3
Reviewed-by: Simon Hausmann <simon.hausmann@nokia.com>