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