Remove variables from the platform-independent GDK API

Remove variables from the platform-independent GDK API

gdk_threads_mutex, gdk_threads_lock and gdk_threads_unlock are removed
from public API. gdk_threads_mutex was deprecated already. Instead of
using gdk_threads_lock and _unlock one was presumably supposed to use
the GDK_THREADS_ENTER and _LEAVE macros, which now simply call the
corresponding gdk_threads_enter() and _leave() functions.

Remove he dllimport/dllexport ugliness for GDK for Windows.

There is still a gdk_display variable being exported by the X11
backend.