Updates and possible inprovements of the libpd backend backend

[mxa]

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Issue to collect possible updates and improvements of the libpd backend (once it is merged).

1. multi-instance support (depends on Merge request https://github.com/libpd/libpd/pull/282 to libpd which depends on WIP merge request https://github.com/pure-data/pure-data/pull/836 to pure-data).
2. use the live-editor from libpd (a Pd instance) to edit the patches, not a separately installed Pd. It currently has issues.
3. evaluate how patches could be executed with blocksize 1 by libpd while CPU load stays acceptable.

one: will be a matter of time, but is going to be implemented. two: should also become implemented but may take a bit more time. three: needs to be discussed.

[mxa]

2. is fixed in libpd and may be implemented in VCV-Prototype