Added technical information regarding clipboard support

Also includes fixes and improvements to the clipboard test harness to
provide more information and work with later versions of FF.
Add support for pasting plain text with line breaks

Pasting plain text with line breaks allows intelligent
merging into lists as well as multiple paragraphs.

Currently does not do the right thing on undo/redo,
as the undo manager does not respect operation groups
(yet).
Pasting data will replace the currently selected content
Add some additional comments about FF’s funny behaviour to the clipboard harness
Renamed large clipboard READMEs to be markdown
Fix copyright headers on PlainTextPasteboard
Clarified Pasteboard.paste to Pasteboard.createPasteOps to be indicate behaviour
Plain text for the clipboard will now (correctly) discard the last new line

By the serializing logic, the selection  <p>text</p> would generate "text\n”.
This is slightly unexpected though, as partially selecting two paragraphs
(e.g., <p>p1</p><p>p2</p>) the user would expect the text to be "p1\np2”.
Easiest way to meet this expectation is to throw away the last new line
if it’s present.
Clean up any carriage returns in pasted plain text

Most applications won’t put this on the clipboard in the first place,
but some Windows ones appear to. Stripping them out before insertion is
the safest way to handle these right now.