Previously, it was not possible to deselect entries by editing the tiddler $:/generated-list-demo-state used in the final example of the SelectWidget docs
This is quite a big change: a new way to invoke action widgets.
The advantage is that it solves #2217 and #1564, a long running problem
that prevented us from adding action widgets to widgets that modify the
store.
This commit adds the new technique for the button and keyboard widgets,
but also extends the select widget to trigger action widgets for the
first time
