Ease the registration of additional properties

[Chris Müller]

To ease the adding of properties (e.g. pending, other vocabularies) to the available schema types, it should be possible to configure them, e.g. in a yaml file:

Brotkrueml\Schema\Model\Type\JobPosting:
  - applicationContact
  - educationRequirements

Brotkrueml\Schema\Model\Type\Event:
  - eventAttendanceMode

• Additional properties can be registered via a yaml file in any extension under Configuration/TxSchema/AdditionalProperties.yaml.
• A slot is available which checks for this yaml file in every extension and registers the given properties to the types.
• The properties are cached.
• The documentation is adjusted.

The yaml file is supplementary to the slot/event listener, as it can be useful to do this programmatically with PHP.

[Chris Müller]

Not necessary by now.