[Core] No longer support links to Node in Links

[alxbilger]

When a path is given to a Link, this path can reference a Node. However, Link are made to reference components. There is an implicit search of the right path based on a path to a Node. It allows to write something like <RigidMapping input="@../" output="@./" />.
IMO, the input and output are not clear in this example. I prefer to explicit the path to the object. That is why I removed the implicit search.
The consequence is that is no longer possible to write <RigidMapping/> (without specifying input and output), because implicitly input was equal to @../, and output was equal to @./ if they were not specified.
That is why it may break some scenes and force to explicit some links.

I also applied changes on mappings to better guide the user.

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Reviewers will merge this pull-request only if

• it builds with SUCCESS for all platforms on the CI.
• it does not generate new warnings.
• it does not generate new unit test failures.
• it does not generate new scene test failures.
• it does not break API compatibility.
• it is more than 1 week old (or has fast-merge label).

[alxbilger]

[ci-build][with-all-tests]