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.
(cherry picked from commit 4a8daf8846)
Add TRACE() calls where appropriate to try to figure out why we are losing CLIPBOARD at times, after transferring PRIMARY to the NSPasteboard.
Use the new pbproxy_clipboard_to_pasteboard where appropriate.
(cherry picked from commit 40190675a6)
