[Ur] Grabbing focus

[Edward Z. Yang]

Hello all,

I have a <dyn> region which at some points in time has a <ctextbox>.  What I
would like is when some code creates the textbox, I would also like it to grab
focus.  Now, I know I can do this with the FFI, but it would be nice if there
was a principled way to do this from Ur/Web.  One may might be to augment the
indexes of the tag constructor; another way might be to add a new
meta-attribute which takes some sort of source and detects when false->true
transitions occur.  One thing to think about is what to do when multiple
things show up and want focus; it's not a very modular way of acting.

Edward