Cleanup x_* to gui_ to match upstream work

Cleanup x_* to gui_ to match upstream work

* ../src/pgtkterm.h (struct pgtk_display_info):

* ../src/pgtkterm.c (mark_pgtkterm, x_free_frame_resources)
(pgtk_update_window_end, pgtk_mouse_position)
(pgtk_redisplay_interface, pgtk_query_frame_background_color)
(pgtk_delete_terminal, pgtk_create_terminal)
(pgtk_query_frame_background_color, frame_highlight)
(motion_notify_event, button_event, scroll_event):

* ../src/pgtkfns.c (x_set_cursor_color, x_icon)
(pgtk_frame_parm_handlers, x_default_font_parameter)
(Fx_create_frame):

"for the time being, commit" -- rename x->gui, frame_X_X(f) macros

ひとまず commit。