Inheritance of visitors

[captainbrosset]

It should be possible at package level to say whether global visitors are inherited or not.

The situation today is:

• if no visitors are configured at package level, global ones apply
• if at least one visitor is configured at package level, global ones don't apply

We want:

• if no visitors are configured at package level, global ones apply only if inherit is not set to false (defaults to true)
• if at least one visitor is configured at package level, global ones don't apply by default, but the sum of all of them do apply if inherit is set to true (defaults to false)