We can now Edit Long Comment and Edit Header Comment.
Changing the number of lines in a long comment exposed a bug in the
display list update. Fixed.
Running the WinForms version trashed the column widths, which exposed
the fact that the default column widths were set too narrow. Fixed.
On an unrelated note, hitting undo/redo quickly will sometimes leave
you scrolled to the bottom of the code list. Some sort of command-
handling race in the WPF framework? (Not fixed.)
Added the progress dialog as a relatively generic thing (it was
implemented as a pair of dialogs in the WinForms version, for no
very good reason). Generation and assembler execution works.
