KFileWidget: reparse config to grab dirs added by other instances of the app
Can't use KConfigWatcher since KFileWidget is short-lived. Well it would have to be a Q_GLOBAL_STATIC, use memory and watch forever even if the user is no longer using the file dialog in this app, not sure the tradeoff is a good one.
With the current solution, I measured that the reparseConfiguration call takes 4ms in kmail (very large app config file), seems acceptable. (Non-optimized Qt build, optimized KF5 build, SSD)