Once you've got to that stage you are honestly better off ditching the processing ide and working with, Eclipse, NetBeans etc that can readily cope with recompiling all source at runtime. You get the benefit of code completion analysis etc, and that more than offsets the loss of the terse syntax of processing.

monkstone is rigth. Actually, I doubt real library development can be done in the PDE, because libraries are written in pure Java, have no setup() / draw(), or other particular code conventions of Processing, so the IDE is almost mandatory (except for my simplistic P8gGraphicsSVG library, which I wrote with a simple text editor, and compiled with a simple shell script... but it has only one source file).