C++: change working-copy to work on UTF-8 encoded QByteArrays.

These not only take less space than UTF-16 encoded QStrings, but due to
the caching in the CppEditorSupport also take less time to build.

This patch also fixes a number of possible encoding issues, where files
and constant strings were (falsely) assumed to be UTF-8.

Change-Id: Ib6f91c9a94ebed5b5dfbd4eb2998825c62c72784
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@digia.com>
Reviewed-by: hjk <hjk121@nokiamail.com>