Make QFontEngine not derive from QObject

Whilst having the objectName set for each engine is
somewhat handy when debugging, deriving from QObject just for that
is a wasting of memory in all other cases.
This also broke the font engine abstraction by allowing qobject_cast()
to access some private data; the only sane way to distinguish
engines is querying their Type value.

Change-Id: Ib1d195692859eb39089f6d8d9016cb8f9dcc0400
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Lars Knoll <lars.knoll@digia.com>