Allow shortcode ui to register default values for attrs

[goldenapples]

Adds an additional field to shortcode attribute data model called 'default_value'. If a default_value is set and the value is empty, the value will be set to the default value.

Can I get a review of this? @jitendraharpalani58 @noppanit @danielbachhuber
This attribute would be useful to me for https://github.com/fusioneng/image-shortcake and I'd like to have it added here if possible. This is the first time I've looked at Shortcake internals and I don't know if this is the best way to do this, though...

[danielbachhuber]

How would you set an empty value?

[goldenapples]

Good question. Probably this needs more thought put into the data structure to hold it.

I was only thinking of the case of selects / radio buttons, where it would be easy to make a value required. Text fields where an empty string is a valid choice, but a default value is still useful, add an extra complication.

[goldenapples]

Closing this. I was misunderstanding the role of the value field, which does something like what I wanted to do here. A better approach is something more like #323 - treating the first value in a select or radio button list as the value if none is otherwise set.