Enable threads in g_type_init()

This means threads will be supported for all gobject libraries/applications
and initialized early enough to not cause any problems.

This solves the problem of libraries needing threadsafety. Previosly
they just called g_threads_init() anyway, which often works but sometimes
breaks in unexpected ways.

See this thread for more details:
http://mail.gnome.org/archives/gtk-devel-list/2009-November/msg00208.html

https://bugzilla.gnome.org/show_bug.cgi?id=606775