Add platform plugin arguments to QLibraryInfo/qt.conf.

Make it possible to specify arguments to the platform plugins in a
section of the qt.conf file:

[Platforms]
WindowsArguments=fontengine=freetype

Change-Id: Ia05d0fa004471dcb74c78a88eec3b220ec3c6ad8
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: David Faure <david.faure@kdab.com>
Reviewed-by: Paul Olav Tvete <paul.tvete@digia.com>