bug#13480: 24.3.50; `C-w' from Isearch should translate newlines to spaces

[Juri Linkov]

> 1. Store in one variable the "raw" (untouched) search string, as
> supplied by the user.

Such variable that holds the raw search string already exists.
It is `isearch-string'.

> 2. Store in another variable (or define a function to get) the
> "canonical" search string, based on the raw search string and the
> "laxities" enabled at that moment for the search algorithm (related to
> whitespace, case-sensitivity, accents, etc).
> 3. Display the canonical search string in the echo area and use it
> also in the search algorithm.

Such variable used to display the search string in the echo area already exists.
It is `isearch-message'.

So we could use `isearch-message' to display the translated search string
but not to translate it in `isearch-string'.