Re-introduced S- into the canonical mappings so that
re-canonicalising will not differ from the original canonicalised form.
Documented recent changes to S- modifier in help.
Refactor events.js to avoid replicating parsing key-strings in two
places. The new fromString is the opposite of toString, and
canonicalisation now proceeds by doing both.
Added the .liberatorShift property to event-representations to allow for
key-combinations that cannot be typed, but can be mapped to. e.g. <S-@>
2d345fc10d
