Change enketo/* overridable modules

[MartijnR]

enketo/config enketo/translator enketo/widgets enketo/dialog enketo/file-manager

To prepare for a future when we can simply load ES6 modules without any bundling (including from node_modules folder), we should change the approach for these placeholder modules.

Maybe add static setter functions like:

• Form.config = ...., Form.dialog = ...

In app.js, we could set these properties with the placeholder files. In an application that use enketo-core, app.js would be replaced by its own entry point.

[MartijnR]

Considering we need those things in many modules (e.g. in a widget), using a static property would mean we need to import Form leading to circular references.

[MartijnR]

Have not been able to come up with a good approach. Will save for later and not include in 5.0.0 release