QPlatformFontDatabase: Add default fonts.

QPlatformFontDatabase: Add default fonts.

Add a default font accessor used to initalize QGuiApplication::font().
Add accessor for default font hash providing the defaults for
QApplication::font(const char *widgetClassName).

Change-Id: Ie4eb6edb251a81e81288ef480ae5550bd1125008
Reviewed-by: Samuel Rødal <sroedal@trolltech.com>
Reviewed-on: http://codereview.qt.nokia.com/2567
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@nokia.com>