Feature: Support AttributeLinkedSet and AttributeLinkedSetIndirect

[jbostoen]

Based on a question from a customer, initial support for AttributeLinkedSet and AttributeLinkedSetIndirect has been added:
cf9cd17

It's only been tested quickly due to time constraints.

Use case:

1. AD user is created
2. This extension creates a person in iTop for this AD user (and keeps some attributes synced from AD to iTop). (Note: on iTop, these attributes are never locked, as it's not a data source synchronization).
3. This extension creates a user account in iTop for this AD user (and keeps some attributes synced from AD to iTop). It's linked to the person object from step 2. It's now also possible to predefine allowed organizations and linked user profiles; but those are only applied upon automatic creation of the user account through this extension.

Not covered (yet):

• If an admin changes the extension's settings in the iTop configuration file afterwards, there's no automatic (un)linking of organizations/profiles.
• If an admin added/removed links in the iTop back-end console, there's no automatic (un)linking of organizations/profiles to match what's in the configuration file.