materialdef files should not contain an absolute path to themselves

For some weird reason materialdef files contain an absolute path to themselves. This makes them painful to share among several developers, as they'll appear modified again and again and again in any version control system solely by opening a project. This can amount to tens of files.

=> make this a relative path (or remove it altogether, the file's path is where you just read it from)

note: in DocumentEditor.cpp, line 2158:

const QFileInfo fileInfo(file);
writeProperty(file, QStringLiteral("path"), fileInfo.absoluteFilePath());