Support for "readonly" and "hidden" visibility modes in ReferenceWidget

[xispa]

Description of the issue/feature this PR addresses

This Pull Request makes the reference widget to support the following visibility settings on "edit" mode: "invisible", "hidden" and "readonly".

Use case

We want the field "Client" to be displayed in read-only mode in Batch edit form when the folder of the context is from "Client" type, so the user cannot change the value:

  <!-- Visibility of Client field in Batch context -->
  <adapter
    factory=".batch.ClientFieldVisibility"
    provides="bika.lims.interfaces.IATWidgetVisibility"
    for="bika.lims.interfaces.IBatch"
    name="MyClientFieldVisibility" />

class ClientFieldVisibility(SenaiteATWidgetVisibility):
    """Handles the Client field visibility in Batch context
    """

    def __init__(self, context):
        super(ClientFieldVisibility, self).__init__(
            context=context, field_names=["Client"])

    def isVisible(self, field, mode="view", default="visible"):
        """Renders the Client field as hidden if the current mode is "edit"
        """
        if mode == "edit" and IClient.providedBy(self.context.aq_parent):
            return "readonly"

        return default

Current behavior before PR

Visibility settings "invisible", "readonly" and "hidden" were not supported in mode "edit".

Desired behavior after PR is merged

Visibility settings "invisible", "readonly" and "hidden" are supported in mode "edit"

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

[ramonski]

Works as expected, thanks!