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Not defining these macros at all causes harsh build breakages.
Better to leave them defined (but documented as deprecated) for now.
Everybody will still get the deprecation warnings for the underlying
gdk_threads_enter/leave.

We can hide the macros again later on when the world has had some
time to port off GDK threads.

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.