API Overhaul: Rename 'widget' to 'field type'

As part of the API overhaul effort per #1041 and #1409, I propose we move away from the somewhat ambiguous term “widget” in reference to field types within the editor. The term widget can be synonymous with sections within a dashboard and as we plan on introducing a dashboard view into our UI, it may be a bit confusing, especially for first-time users.

We currently call them fields within the config file. So I'd imagine it may be more clear if we changed widget to type in each field in the fields array to avoid confusion in the future.

Likewise instead of `registerWidget` we might rename this `registerFieldType` making way for `registerDashboardWidget`.