bug#6225: Please add a hook for `isearch-process-search-string'

[Lennart Borgman]

There is AFAICS no way to currently trigger actions when isearch
search string changes.

I think it would be useful to have a hook for this. Some things that
you could do would for example

- Sync with external apps (I currently want to sync with display in Firefox).

- Show matches in buffers displayed in other windows on the same frame
could sometimes be useful.


Could we please add such a hook? I think the place to run it is in
`isearch-process-search-string'.