Restructure widget folders

As our widget library grows we should consider creating a better folder structure for them.
This is particularly important now that we are adding multiple button and input types. There are many ways we could go about grouping our widgets.

**My initial thoughts are thus;**

1. Group all `form` widgets under a `form` directory. (`form/Select`, `form/Text` etc)
2. Group `button` widgets under a `button` directory. (`button/Button`, `button/Raised` etc)
3. Drop barrel files (index.ts)

**Questions**

1. Should we group `inputs` ie, `Text`/`Number`/`Email` etc under a `form/input` directory?
2. Should we group `selects` ie. `Select`/`Combo`/`Typeahead` under a `form/select` directory?