[Ur] New feature round-up

[Adam Chlipala]

In response to some requests by Edward Z. Yang (seconded by a few 
others) about 2 months ago, I've added the following features, and they 
are available in the public Mercurial repository.

- Mouse and keyboard event handlers are now passed records of various 
event information, including the status of the shift key and others.  
(Note that this may require syntax changes in old code using such event 
handlers, especially for mouse event handlers, which previously were not 
functions.)
- The function [Basis.giveFocus] makes it possible to give a tag focus 
by ID.
- A new pseudo-tag allows embedding of a [transaction] in HTML, where 
this code is run to determine what HTML should go in that position, 
possibly first allocating [source]s or causing other side effects.  Use 
like: <active code={return <xml>Hi!</xml>}/>

I'll be curious about what folks think about these new features.  I'd 
like to put out a new Ur/Web release soon, with these features and any 
other small fixes that are reasonable to do in the next week or so.