Add support for QRawFont in QTextLayout.

Add support for QRawFont in QTextLayout.

The purpose of this change is to allow shaping complex
text while using QRawFont.
This is needed for WebKit so that we can switch to
using QRawFont everywhere and be more in line with
what other WebKit ports do.
Since this change slightly bends the aim of QRawFont,
let's reserve this for internal use for now.

Change-Id: I5ec0881f50ce288350bd277570cb5f1fb70c355c
Reviewed-by: Simon Hausmann <simon.hausmann@nokia.com>