Consistent AU factory Preset IDs

Consistent AU factory Preset IDs

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 ae4604a24b7), simple
sequential numbering is no longer an option.