Add reducer for registering default data

[edgarmueller]

Introduce a new reducer for adding a substate that keeps track of any pre-defined data that can be queried by clients. Currently, the only such client is the AddItemDialog which reads the registry and merges any result with any default values found in the schema, if such.
Registration is happening via a given schema path and a JSON value. No validation of the registered data or whatsoever is taking place.
The material-tree-renderer example has been updated to demonstrate this new feature.

[coveralls]

Coverage increased (+0.4%) to 89.204% when pulling dde82a0 on edgarmueller:topic/add-item-dialog-default into 64fb88f on eclipsesource:master.

[eneufeld]

Looks good, would like to discuss pros and cons of the suggested solution though

[eneufeld]

I suggest to merge this and create a new issue in order to discuss how we can improve the way default data can be provided to jsonforms.

[edgarmueller]

@eneufeld Sounds good, could you approve the PR then, please?