With this, display modes and tree elements can add an element through
the tree-display object used for building the tree for this display
mode. This means we no longer have to pass the `SpaceOutliner` object
around, and we avoid boilerplate. Further such simplifications to the
tree element creations are planned and are easier now.
Also note that this centralizes the use of `SpaceOutliner`, so we can
more easily make tree-display and tree-element objects independent from
it. This could be useful for unit testing, for example.
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