Drop WGET and revert read access to Lisp_Objects slots of struct window.

Drop WGET and revert read access to Lisp_Objects slots of struct window.
* window.h (WGET): Remove.
(struct window): Do not use INTERNAL_FIELD.
* alloc.c, buffer.c, composite.c, dispextern.h, dispnew.c, editfns.c:
* fileio.c, font.c, fontset.c, frame.c, frame.h, fringe.c, indent.c:
* insdel.c, keyboard.c, keymap.c, lisp.h, minibuf.c, msdos.c, nsfns.m:
* nsmenu.m, nsterm.m, print.c, textprop.c, w32fns.c, w32menu.c:
* w32term.c, window.c, xdisp.c, xfaces.c, xfns.c, xmenu.c, xterm.c:
Adjust users.