Don't trim escaped quotes

Messages in .po files are sometimes split over several lines. In the conversion to make strings.php, quotes and newline characters are trimmed and the lines are merged. However, in the case when there happen to be an escaped quote last in the line (\""), both quote characters are trimmed, and only the backslash is left. I have seen this at least when a .po file is fetched from Transifex "for use".

Example:
> "The web installer needs to be able to create a file called \".htconfig.php\""
> " in the top folder of your web server and it is unable to do so."

in the .po file is converted to

> "The web installer needs to be able to create a file called \".htconfig.php\ in the top folder of your web server and it is unable to do so."

in strings.php.