Allow storing libraries in subfolders of the libraries folder

[seachord]

I currently have well over a hundred libraries accumulated. I'm sure many active users have even more! It getting clumsy to sort thru them by eye! It would be very helpful if there was a way to have a hierarchical menu system so Libraries could be sorted into folders with meaningful names, and easier to find.

I suspect this might make it harder/slower for the libraries to be found by the IDE. But the the vast wealth of libraries available - it might be worth it!

[matthijskooijman]

A long time ago I already implemented this, suffering from the same problem. See arduino/Arduino#1986. IIRC that stalled because people could not agree on how to present this in the UI, and by now I believe the PR is probably completely outdated (but might be revamped if there was interest).