Senaite labels

[ramonski]

Description of the issue/feature this PR addresses

This PR introduces labels for SENAITE:

Basically all AT/DX contents can be configured to have labels via the SENAITE registry, unless they define their schema solely over a behavior, e.g. a DataBox.
In this case, the extended label field does not appear in the edit view, but the contents can be still labeled programmatically with the provided Label API.

The extended label field queries existing labels that have been created in the "Labels" setup folder, but allow also to enter arbitrary labels for a specific object.

Object labels are stored in the annotation storage to avoid attribute clashes and define extend the following field to the object:

• AT Content Types: ExtLabels
• DX Content Types: ext_labels

When a label is assigned to an object, the object will be cataloged in the senaite_catalog_label catalog.

The labels index allows to search for all labeled objects in SENAITE.

Current behavior before PR

No labels available

Desired behavior after PR is merged

All objects can be labeled

--
I confirm I have tested this PR thoroughly and coded it according to PEP8
and Plone's Python styleguide standards.