scope: drag and drop (DnD)

[ssj71]

lv2#1
provides a drag and drop interface for pugl. Comparable backends like gdk provide DnD but for plugins DnD isn't typically useful.

I think we want cross-platorm feature parity before adding API. I haven't reviewed the proposed change yet. If its API is consistent and sensible and we can add support for macOS and Windows then I don't have a real problem with merging it.

We can certainly discuss further.

[drobilla]

That implementation aside, one major advantage of DnD comes to mind: it would often let you avoid the file selector problem, which would be quite nice...