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