Rebuilding the tree immediately after changes could cause the tree to be
rebuilt multiple times. More importantly, it made it harder to reason
about thread safety, since we would touch the tree within a whole bunch
of API functions. Now tree building is simplified and managed in a
single place, so making the tree building thread safe can be made
trivially in a follow-up.
Note, this means the initial catalog tree building doesn't happen in a
background thread together with loading the asset library and catalogs
anymore. But we would already do all further rebuilds on the main thread
anyway, this shouldn't have any notable impact.
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