Use INTERNAL_FIELD for windows.

Use INTERNAL_FIELD for windows.
* src/window.h (WVAR): New macro.
(struct window): Change Lisp_Object members to INTERNAL_FIELD.
* src/alloc.c, src/buffer.c, src/composite.c, src/dispextern.h:
* src/dispnew.c, src/editfns.c, src/fileio.c, src/font.c, src/fontset.c:
* src/frame.c, src/frame.h, src/fringe.c, src/indent.c, src/insdel.c:
* src/keyboard.c, src/keymap.c, src/lisp.h, src/minibuf.c, src/nsterm.m:
* src/print.c, src/textprop.c, src/w32fns.c, src/w32menu.c, src/w32term.c:
* src/window.c, src/xdisp.c, src/xfaces.c, src/xfns.c, src/xmenu.c:
* src/xterm.c: Users changed.
* admin/coccinelle/window.cocci: Semantic patch to replace direct
access to Lisp_Object members of struct window to WVAR.