This allows us to update the jump label positions when the viewport
changes.
This in turn allows us to stay in URL mode while the user is using the
mouse to scroll in the scrollback history.
Scrolling with the keyboard is currently not possible, since input
handling in URL mode does not recognize “regular” key bindings. We
_could_ add scrollback up/down bindings to URL mode too, but lets not,
for the time being.
(Note: an alternative to this patch is to disallow mouse scrolling
too. Then we could have kept the URL start/end as viewport local
coordinates).
This works just like show-urls-launch, except that instead of opening
the URL (typically using xdg-open), it is placed in the clipboard when
activated.
While parenthesis and brackets _are_ valid URL characters, there are
many times when we do *not* want them to be part of the URL.
For example, in markdown we write “[text](url)”, or even
“[](url-2)”.
Here, the URLs are clearly *not* “url)” or “url-1)](url2)”.
