Improve behavior of help popups

* I was encountering an issue in which the popups would disappear before
  you leave the menu item.  I fixed it by showing them on mouseover as well
  as on mouseenter.  (I think the mouseenter was necessary for some cases.)
* I cleaned up the code a bit along the way.
Revise MISTui menu library

* Use standard CSS rather than JQuery as it seems to give a better
  experience (no menus left hanging).
* Add a simple experiment.
Minor updates to mistui-expert.

* Got rid of some experimental code (name "fred").
* I think I got some bug fixes done, too.
Update expert GUI to use new menu system.
Add basic builtin functions.

* Also update mist-functions.js to support better function info.
Update docs for builtin functions.
Add menu items for builtin functions to expert UI.
Add support for parsing numbers.
Correct error messages in MIST.parse.
Make menus close when clicked.

* Add a "disabled" style.
* Add handler to set the menu to that style.
Add simple expression typing.
Permit periods when parsing identifiers.
Minor improvement to menus

* Menus hide better on click.
* But still not on Safari/Mac.
Add basic rendering library for the new function model.
Add basic MIST animation library.
Update expert UI to render/animate.
save.php saves images
Correct types in save-expt.html
Draft save/restore workspace in expert ui.