DirectShow: Sanitize DirectShowMediaType

The DirectShowMediaType is now a thin data wrapper around AM_MEDIA_TYPE
and will for the most look and work the same way, with the exception
that it cleans up after itself. All utility functions are now static,
except clear() which, for convenience, is also provided as a non-static
member function.

In addition to the changes mentioned above, duplicated methods were
removed, conversion for ARGB32 was added, and an attempt to make
the usage of AM_MEDIA_TYPE and DirectShowMediaType was made more
consistent.

Change-Id: Iad32fb8eeabd9d4183e9bd10426cac3963e5d99a
Reviewed-by: Yoann Lopes <yoann.lopes@qt.io>