Use system codepage when reading from piped command

Summary:
When using a piped command to preprocess the message the response may contain locale-specific characters. When converting using Latin1 those characters are mangled.

This fixes preprocessing messages which contain Unicode characters.

Test Plan: Use a piped command that generates non-ASCII characters in the response. Make sure that these characters appear properly in the editor window.

Reviewers: dvratil, mlaurent, #kde_pim

Reviewed By: dvratil, #kde_pim

Subscribers: kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D19333