Fix critical color selector bug

It was possible to click-drag picture-box-based controls beyond the
edges of the picture box, leading to RGB values like -5.  Oops.
Start of work on new Print dialog

I've got a pretty great UI in mind for this, so fingers crossed that it
works out as planned.  I think PD may finally make image printing a
non-miserable experience...

Of course, this commit does not do anything interesting yet.  Just some
nasty API work to retrieve supported paper sizes from the printer.