each in own file
list of components component
switch to new firstNode/lastNode hint system

concept of "heavyweight" components didn't work.
appended components and DOM would just go outside
the comments.  can't change the meaning of firstNode/
lastNode that way.
don't invite rebuilding components that don't expect it
else case goes in UI.List

I figure it's central enough, might be animated.
Switching between items and else shouldn't be
a rebuild, and it definitely shouldn't affect the
cursor observer.
reactive observing #each