codeview: Don't link to not found files
The source path inside po files is relative to the project root. If you open the po file placed inside a gnome project this should work, but you can open a po file placed anywhere so instead of show a broken link, it's better to show the path as plain text.
This patch tries to find the source file in ../PATH relative to the .po location.
A new function called find_source_file has been added and can be improved to try to find the source file.
See #100