customize-create-theme

[Chong Yidong]

I've checked in a new interface for M-x customize-create-theme into
cus-theme.el.  Hopefully, this should be easier to use than the old
one.  Also, I've removed a couple of unused code paths in the Custom
Themes code (which were apparently adapted willy-nilly from XEmacs),
so the code should be a little more understandable now.

Those of you who've had trouble using or reading the Custom Themes
code may want to try again.  Bug reports welcome.

(The old customize-create-themes interface had a couple of pretty
obvious bugs in it -- one of them due to me -- but no one seemed to
notice.  Oh well.)