Only import the parts of libraries we need.

According to the [edicts of Processing](https://github.com/processing/processing/wiki/Library-Basics).

Fixes [#87](https://github.com/jdf/Processing.py-Bugs/issues/87).

(Also, while working on this I noticed that we recreate LibraryImporter every time we run a sketch. This means that we re-add stuff to the various paths - it doesn't duplicate anything, but it does take a second or two with a slow hard drive. Should I change that?)
This function doesn't know that it's a "fallback"; this comment should go to the call site.