Don't overuse 'const' in types of locals.

Don't overuse 'const' in types of locals.

* bidi.c (bidi_count_bytes):
* gtkutil.c, gtkutil.h (xg_event_is_for_menubar)
(xg_event_is_for_scrollbar):
* xselect.c (x_handle_property_notify)
(x_handle_selection_notify, x_handle_dnd_message):
* xsettings.c, xsettings.h (xft_settings_event):
* xterm.c (x_handle_net_wm_state, handle_one_event)
(x_menubar_window_to_frame, x_detect_focus_change)
(construct_mouse_click, note_mouse_movement)
(x_scroll_bar_to_input_event, x_scroll_bar_expose)
(x_scroll_bar_handle_click, x_scroll_bar_note_movement)
(handle_one_xevent, x_handle_net_wm_state):
* xterm.h (x_handle_property_notify, x_handle_selection_notify)
(x_handle_dnd_message):
Avoid unnecessary 'const', typically the second 'const' in
'const foo * const arg', a 'const' that does not affect the API
and doesn't significantly help the human reader.