Fix minor GTK3 problems found by static checking.

Fix minor GTK3 problems found by static checking.

* emacsgtkfixed.c (EMACS_TYPE_FIXED, EMACS_FIXED, EmacsFixed)
(EmacsFixedPrivate, EmacsFixedClass, struct _EmacsFixed)
(struct _EmacsFixedClass, emacs_fixed_get_type):
Move decls here from emacsgtkfixed.h, since they needn't be public.
(emacs_fixed_get_type): Now static.
(emacs_fixed_class_init): Omit unused local.
(emacs_fixed_child_type): Remove; unused.
* emacsgtkfixed.h (EMACS_TYPE_FIXED, EMACS_FIXED, EmacsFixed)
(EmacsFixedPrivate, EmacsFixedClass, struct _EmacsFixed)
(struct _EmacsFixedClass): Move to emacsgtkfixed.c.
(EMACS_FIXED_CLASS, EMACS_IS_FIXED, EMACS_IS_FIXED_CLASS)
(EMACS_FIXED_GET_CLASS): Remove; unused.
* gtkutil.c (xg_create_frame_widgets) [!HAVE_GTK3]: Omit unused local.