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This commit deprecates gdk_threads_init, gdk_threads_enter,
gdk_threads_leave and gdk_threads_set_lock_functions. Using GTK+
from multiple threads does not work at all on Windows, and is
problematic on other platforms as well. We want to move to a world
where all GTK+ calls are made from the main thread.
Use g_main_context_invoke, g_idle_add and related functions if you
need to schedule GTK+ calls from other threads.

http://bugzilla.gnome.org/show_bug.cgi?id=680754

Move declarations to other headers, and introduce some new ones:
gdkmain.h, gdkrectangle.h, gdkthreads.h.