Fixed thread performance on SteamOS. Fixed Runner.default_path and AddGameDialog.

Downloading and install games with Lutris was ridiculously slow on SteamOS but it worked fine on Fedora for some reason; the speed that files were extracted was the biggest problem. I took some time to find the root problem and I found a solution. Due to the use of threading in Lutris, it looks like calling GObject.threads_init is required before calling Gtk.main.

After making that change, downloading and installing games worked as expected. I added a fix for Runner.default_path. Also, I found out that the recent changes to the configuration system broke adding games using AddGameDialog. That issue has been fixed as well.