Fixed thread performance on SteamOS. Fixed Runner.default_path.

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.

http://faq.pygtk.org/index.py?req=edit&file=faq20.006.htp

After making that change, downloading and installing games worked as expected. Also, I added a fix for Runner.default_path.