Re: ACTION-2196: Spec up collapsing sections

As a reminder, this action is waiting for us to make a decision about how  
we best want to deal with collapsing sections.

So to summarise:

Requirements:

1. The simple case should be really easy: just adding an attribute value
makes a group that is collapsible/openable with a simple interaction.

2. It should be possible to enquire what the state is (Open/closed)

3. It should be possible to change the state.

Options:

1. Use values, or MIP states of @ref to signal collapse/open

2. Use appearance="{expression}"

3. Actions and/or Events: Use an action or send an event to toggle the  
state, and use events to announce state change.
    For instance, we could repurpose the actions and events around dialog  
to work here as well:
       <hide dialog="groupid"/>
       <show dialog="groupid"/>
       xforms-dialog-hide/xforms-dialog-show
4. Other possibilities?

Steven