Investigate plugin fx gui embedded in the track mixer

[abique]

Suggested with https://github.com/justinfrankel/reaper-sdk/blob/main/sdk/reaper_plugin_fx_embed.h as a source of inspiration.

[joshka]

I think this might be similar to my suggestion over at #206 which talks about the way someone wrote a Valhalla Max4Live device that puts the UI in Ableton's device panel. If so, +1 to this idea :)

Something that Max4Live seems to do well for Ableton is be able to make user interfaces that fit within the daw rather than floating over it. This enabled Elisa Hom to create a M4L device that presents a really nice interface for the Valhalla plugins.

I wonder if this might be a feasible goal for a future version of clap - I don't see this being obviously possible in gui.h. I'd love to see more plugins do something similar to the above for Bitwig (and any other DAW).

Implementation-wise, I'd imaging that this would be something akin to the DAW communicating constraints on the size (max height / width) of the window, allowing the plugin to perhaps include different UX for each DAW size.

I'm not a plugin dev, so there isn't a specific use case for this other than wanting to see it for plugins I'd use.