Minor cleanup to avoid forward declarations.

Minor cleanup to avoid forward declarations.
* coding.h (struct ccl_spec): Remove forward declaration.
* composite.h (toplevel): Include font.h.
(struct composition_it, struct face, struct font_metrics):
Remove forward declaration.
* dispextern.h (struct image, struct atimer): Likewise.
* emacsgtkfixed.h (struct frame): Likewise.
* emacsgtkfixed.c (toplevel): Reorder headers and drop stdio.h.
* font.h (struct font_driver, struct font, struct glyph_string)
(struct face): Remove forward declaration.
* fontset.h (struct face, struct font): Likewise.
* frame.h (toplevel): Style cleanup.
(enum output_method): Move to...
* termhooks.h (enum output_method): ...here.
(struct glyph, struct frame, struct ns_display_info)
(struct x_display_info, struct w32_display_info):
Remove forward declaration.
* xterm.h (toplevel): Include termhooks.h.
(struct font, struct window, struct glyph_matrix, struct frame)
(struct input_event, struct face, struct image): Remove forward
declaration.
* gtkutil.h (struct _widget_value): Likewise.
* keyboard.h (toplevel): Include termhooks.h.
(struct input_event): Remove forward declaration.