Creator should mark build folder files as readonly

From a comment on QTBUG-127661:

I recommend that Qt marks the files in the build output directory read-only for owner, so that they cannot accidentally be changed, too.

It would be nice if Qt Creator could mark files from the build folder in the editor, or mark them as readonly, to avoid users from editing autogenerated files. Its a bit annoying to edit a file in the build folder by mistake and getting all your changes overwritten during the next build.