Add experimental tree builder classes

These classes provide mechanisms to build a traversable tree of pages
and links. Either from the existing namespace structure, or from a
control page containing

Add experimental tree builder classes

These classes provide mechanisms to build a traversable tree of pages
and links. Either from the existing namespace structure, or from a
control page containing (possibly a nested) set of links.

The nodes returned by the tree are deliberately sparse. No ACL checking
is taking place. Developers can enrich (or omit) nodes and influence
recursion decisions via callbacks.

The tree can optionally be sorted by comparators provided in the
TreeSort class or a custom callback.

The API provided by these classes is not considered stable yet and may
change over time. Plugin authors are encouraged to use them and provide
feedback.

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