Add support for multiple UIs

[mxmilkiib]

Through Synthpod, Moony.lv2 can have multiple UI windows per instance. Might this be possible Jalv?

[ventosus]

I've made a PR some years back for this [1]

@drobilla should I refactor the patch, is there still interest for this ?

[1] https://github.com/ventosus/jalv/commits/multiple-uis

[drobilla]

Sure. All hosts probably need to get away from making single-ui-per-type assumptions...

[drobilla]

I'm assuming this means selecting from multiple UIs, not displaying several UIs at once (which seems out of scope for Jalv to me). That was added in 6721ea5.