Prevent crashes from Lisp code modifying xwidget-list

Prevent crashes from Lisp code modifying xwidget-list

* src/xwidget.c (internal_xwidget_view_list)
(internal_xwidget_list): New variable.

(find_xwidget_for_offscreen_window)
(define_cursors, offscreen_damage_event)
(webkit_ready_to_show, xwidget_init_view)
(Fxwidget_resize, Fdelete_xwidget_view)
(Fxwidget_view_lookup, xwidget_spec_value)
(lookup_xwidget, xwidget_end_redisplay)
(kill_frame_xwidget_views, kill_buffer_xwidgets)
(Fmake_xwidget, Fget_buffer_xwidgets): Use internal list.

(syms_of_xwidget): Initialize internal xwidget lists.