Revert "First take on a Constrainable interface (not used by any other interface yet)."

This reverts commit 04f096b11ea7c74a827c1c1eaded849bd5354cd1.

Conflicts:

	getusermedia.html
Revert "MediaStreamTrack implements Constrainable."

This reverts commit f93ea1d0c6c2ce36723e0cef02730fb686b33db1.
Added clarifying text about MediaStreamTrack lifecycle and mediaflow.
Fixed broken end tag (that messed up the syntax highlightning in my editor).
Split up MediaStreamTrack section into sub-sections (content unchanged).
Made track cloning explicit.
WebIDL clean up (list feedback).