Deprecate GStaticPrivate and g_thread_foreach

This commit moves GStaticPrivate, g_thread_foreach and all
related functions and variables to gthread-deprecated.c. We
introduce some internal API to make this possible.

g_thread_foreach is not a very useful function, since there is
virtually nothing you can do with a GThread*, and implementing
it requires us to keep a list of threads around.

GStaticPrivate has been made redundant by adding comparable
capabilities to GPrivate.

https://bugzilla.gnome.org/show_bug.cgi?id=660635