Why does it need to be broken up into multiple files?
It doesn't "need" to be broken up in multiple files, but like any other larger software project I want to structure it and be able to reuse code. Besides that, the Arduino IDE is really too basic to work on large files. I hate to scroll back and forth all the time.
You can have two pde files in one sketch directory. Opening the sketch will open both pde files, in separate tabs. You can put functions in one pde file, and call them from the other pde file.
I know, but I tried it with classes and that didn't work (like in Processing). I guess it's because it's not Java. Still I would expect to be able to #include a header from the same folder as my sketch, have its cpp file compiled referencing the same libraries that my main sketch is able to use.
So really there is no way to create classes that use libraries, and (re)use those classes in a sketch?