Don’t export C symbols not used elsewhere

Don’t export C symbols not used elsewhere

These were recently added, mostly as part of xwidget code.
* src/emacsgtkfixed.c (emacs_fixed_get_type): Now static.
(EMACS_FIXED, EMACS_FIXED_GET_CLASS):
Now static functions here, not macros in emacsgtkfixed.h.
* src/emacsgtkfixed.h (EMACS_TYPE_FIXED):
Remove.  All uses replaced by definiens.
(EMACS_FIXED, EMACS_FIXED_GET_CLASS):
Remove; these are now static functions in emacsgtkfixed.c.
(EMACS_FIXED_CLASS, EMACS_IS_FIXED, EMACS_IS_FIXED_CLASS):
Remove; unused.
(emacs_fixed_get_type): Remove decl; no longer extern.
* src/xwidget.c (offscreen_damage_event)
(webkit_mime_type_policy_typedecision_requested_cb)
(webkit_new_window_policy_decision_requested_cb)
(webkit_navigation_policy_decision_requested_cb)
(xwidget_spec_value, xwidget_view_lookup)
(xwidget_start_redisplay, xwidget_touch):
Now static.
* src/xwidget.h (xwidget_start_redisplay, xwidget_touch):
Remove decls.