Add rich text formatting to comments.

This is still a bit experimental, but for starters it works reasonably
well already. Formatting is done by adding certain formatting tags to
the input, please check the help patch for details. Note that there's no
WYSIWYG editing of the formatting at present, this may still be added in
the future.

Obviously, this won't replace your word processor anytime soon, and
entering all the formatting tags can get a bit tedious without any GUI
support. But at least it provides you with some useful formatting
options for comments to improve their readability and make them visually
more appealing.

The same caveats apply as with explicit line breaks in comments. That
is, none of the formatting tags will work in vanilla Pd at present,
instead you will see the text as shown while editing the comment box.
Which doesn't look great, TBH, thus rich text formatting is best avoided
in patches that you plan to use with vanilla Pd.