Move libProcessing into its own repo

[catilac]

Given the ideas floating around python, and the nature of libProcessing itself, it would make sense to move that code into its own repository. Additionally it should be brought into the Processing4 effort through a git submodule (for now)

Another added benefit besides modularity, would be a CI system that could run focused entirely on the work itself.

[catilac]

@tychedelia https://github.com/processing/libprocessing -- how does this look? I'm going to see about moving my PR commits to a PR for this repo now. Or I might just recreate them here.