add "presets_updated" notification function in aap_presets_host_extension_t

[atsushieno]

There should be some way for an AAP (plugin) that has updated presets list (either new or deleted items, or some content in the list) to send notifications to the host. Otherwise host will have to query preset count every time it needs to get, and there will be extraneous interactions between the host and the client.

[atsushieno]

On a related note, Compose Native UI holds presetCount as a member of PluginViewScope which is currently not observable (raw Int). That likely needs to change.

[atsushieno]

It is addressed at c433720.