Tree: Add missing tree impl. switch (sponsored by Mrs. Jansen)

[mjansenDatabay]

This PR adds an Achievable Setup Objective to let administrators switch the Tree Implementation of the repository tree from a Nested Set implementation to a Materialized Path implementation and vice versa.

Debatable: Should we stick with the suggested objective class or should we segregate this into two separate objectives?

Using ilTree to migrate to Nested Set was not possible because of massive global scope usage, so I had to duplicate a few lines of code.

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Furthermore this PR adds a Metrics Collected Objective to report the current tree implementation when executing the status command of the ILIAS setup.

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• The builds fail because of the missing license header in dbupdate_05.php.
• If approved, this should be picked to release_8 as well.

Mantis Issue: https://mantis.ilias.de/view.php?id=32277