XQuartz: pbproxy: pbproxy now responds to selection request events.

We can now copy and paste images to and from X11.  Text copying and
pasting works as well.

The NSPasteboard can contain TIFF or PICT images, and pbproxy will
translate to an image/png or image/jpeg request, and list those in the
TARGETS.

I added a description of the basic design at the top of x-selection.m.

I removed the request_data x_selection class struct.  It's not needed.