The dash symbol in the widget id

[katspaugh]

There seems to be a bug with widgets that have a dash in their id.

I created a custom widget with the id design-preview and used it in a newly created content type. When I tried to save the content type, it gave me an error. Looking at the network panel of Chrome's dev tools revealed, that the server responded with an error: Incorrect widget. Id doesn't match the regex [A-Za-z0-9]. When I changed the id of the widget to design, I was finally able to save the content type.

Since some of the examples in this repo have a dash in their id, I suggest this symbol should be allowed in the widget id.

[katspaugh]

Duplicate of #86.