[XForms 2] New feature - external models

Kicking off the future version discussion. Here is the first of the  
features I would like to see.

External models

	<model id="tax" src="taxes.xml" />

This is above all useful for making several forms for editing a model, so  
that you don't have to keep updating the models all over the place as the  
details change. Binds can be used to shield the forms from internal model  
changes.

I'm not sure that much has to change to support this, though we need to  
explicitely address the question of whether it is still ok that @bind is  
an idref

	<input model="tax" bind="income">...

Maybe in XForms 2, we could change the use of IDREF into something else,  
so that we are not restricted in the same way as we have been (thereby  
making the switch in repeat problem go away).

Steven