Use AU's preset->presetNumber as identifier since std::map are sorted
this also indirectly sorts presets by preset-number. (user presets
start with a '/' and are listed first, sorted by name).
Since Presets are now identified by URI on session load (53a0199a0)
and AU user-presets can added/be removed (since ae4604a24b), simple
sequential numbering is no longer an option.
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