Add Panels integration to the D6 version

Panels for D6 has an early, still buggy, release out. There are lots of problems still, but the integration is supposed to be the same for the D5 and D6 versions, so the .inc files we created for D5 should more or less work correctly in D6. I tried testing this out and Panels is still too buggy for me to be sure, but I think our integration code will mostly work.

I have a patch ready to add the panels.inc back into the D6 version so people can start using it with Panels and post issues about things that still need to be fixed. I'm planning to commit this shortly unless someone (yched?) sees any reason not to.