Due to the work on gdk_cursor_new_from_surface (commit b2113b73),
get_cursor_for_pixbuf() in GdkDisplayClass was converted to
get_cursor_for_surface(), which means the GDK Win32 backend needs to be
updated for the code to build and run on Windows, plus some function
prototypes and declarations/calls need to be updated as well.
https://bugzilla.gnome.org/show_bug.cgi?id=705980
Move all the system includes, defines and function prototypes into a
separate header gdkwin32misc.h, so that we could keep gdkwin32.h as simple
as possible.