include: Convert vfw.h to vfw.idl.

[Nikolay Sivov]

Hi, I see it went to pending, so let me suggest something.

We need this idl mostly for automated proxy/stub generation and 
registration, right? Maybe we could introduce an idl that contains 
interfaces only and not all regular header stuff that vfw.h is made of. 
I would probably work to duplicate interfaces locally at dll level, and 
leave header alone.