CppTools: Use editor manager's codec as fallback

...for reading not already opened files.

This partly reverts commit f7c68f6b. In case TextFileFormat::detect()
fails, the user configurable editor manager's codec is used instead of
QTextCodec::codecForLocale().

Adds also a qWarning() to easier detect encoding errors.

Task-number: QTCREATORBUG-10378

Change-Id: I0fa4e6b898ed090d85414ce2a001f11b115a42d3
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: Eike Ziller <eike.ziller@digia.com>
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>