Constraints as a separate interface

Here's an attempt to separate constraints out into a separate interface.   The point is to allow other specs, such as MediaRecorder, to refer to it and ensure that they are all using constraints consistently.  This does _not_ change the semantics of constraints, it would just break them out into a separate interface.  This interface would still be defined in the MediaCapture spec, but in a separate section.  (There would be editorial changes throughout the spec to refer to this interface, but, again, the semantics of Tracks and Streams wouldn't change.)


-          Jim