We weren't escaping '<', '>', and '&', which caused browsers to get
very confused. Browsers seem to prefer <PRE> to <CODE> for long
blocks of text, so switch to that.
Also, added support for putting long labels on their own lines in
the HTML output.
Also, fixed some unescaped angle brackets in the manual.
Also, tweaked the edit instruction operand a bit more.
This feature "exports" the source code in the same format it appears
in on screen. The five columns on the left are optional, the four
on the right can be resized. Exported text can span the entire file
or just the current selection. Output can be plain text or CSV.
(I still haven't figured out a good use for CSV.)
The old copy-to-clipboard function is now implemented via the export
mechanism.
This adds a new clipboard mode that includes all columns. Potentially
useful for filing bug reports against SourceGen.
Ported the column width stuff from EditAppSettings, which it turns
out can be simplified slightly.
Moved the clipboard copy code out into its own class.
Disabled "File > Print", which has never done anything and isn't
likely to do anything in the near future.
Also, added a note to 2019-local-variables about a test case it
should probably have.
