Undo deletes initial contents and related undo issues

[NSoiffer]

Description

This is possibly related to #139...

On the undo stack is (apparently) the initial contents of the editor. I don't believe that is correct. This leads to some things that seems wrong.

Overall, I get the feeling there is an off-by-one or fence post error going on.

Steps to Reproduce first issue

1. Start with mathlive/examples/basic/index.html. This starts with f(x)=
2. Click after the =. Type ctrl-z/undo. The entire expression disappears. This seems wrong -- nothing should happen since there is nothing typed to undo.

Continuing on with this example:

1. Type ctrl-y/redo -- nothing happens, even if typed repeatedly. Of course, if the bug above didn't happen, that would be the expected behavior.

Steps to Reproduce variation on first issue

1. Start with mathlive/examples/basic/index.html. This starts with f(x)=
2. Click after the =. Type a
3. Type ctrl-z/undo. The a goes away. Good.
4. Type ctrl-z/undo. The entire expression disappears (as above).
5. Type ctrl-y/redo -- nothing happens. Bad. But, typing ctrl-y/redo again brings back f(x)=. However, typing it more times never makes the a show up again.