Support readonly defined in JSON Schema and UI Schema

[sdirix]

Feature

• JSON Schema specifies a readOnly attribute
• The Angular renderers support options: { readonly: true }

We should support both of these in the JSON Forms Core, so that in all renderer sets enabled respects not only the state-global-readonly and rules, but also the JSON Schema and UI Schema settings.

Describe for which setup you like to have the improvement
Framework: [core]

[sdirix]

Suggestion

Determine the enabled flag by following these steps.
Whenever enabled is determined, skip the remaining steps.

1. If the form-wide readonly flag is true => enabled: false
2. If there is an ENABLE or DISABLE rule for the current ui schema element => set enabled accordingly
3. Check if uischema.options.readonly is set. If yes, set enabled accordingly
4. Check if uischema.options.readOnly is set. If yes, set enabled accordingly
5. Check if config.readonly is set. If yes, set enabled accordingly
6. Check if config.readOnly is set. If yes, set enabled accordingly
7. Check if schema.readOnly is set to true. If yes set enabled: false
8. Set enabled according to the parent's enabled (i.e. check ownProps.enabled).

These modifications need to be part of JSON Forms core. For example in mapStateToControlProps, but also all other mapStateToXZY. At the moment the core only checks for case 1, 2 and 8. See here.

There should be unit tests.
There should be a new example in examples, covering all cases (except case 1).
Readonly needs to be documented on the JSON Forms Website.

The Angular Material renderers also need to be modified. They currently check themselves for case 3. This is then no longer necessary. They should just take the determined value from the core.

[sdirix]

Implemented with #1763