This allows easy re-use of commands, tying names
to URL-regexes rather than command-strings.
Much more user-friendly, I reckon!
Also, now we use “$URL” instead of “%s” for URL
substitution in commands. More shelly that way.
Now we don’t run a hardcoded command when the
address-bar button is pressed, but the command
configured for that URL.
Also, we only enable the address-bar button for
tabs whose URL is matched by some command.