tui: replace deprecated_register_changed_hook with observer

tui: replace deprecated_register_changed_hook with observer

This is a straightforward replacement of the TUI's use of the
aforementioned hook with the register_changed observer.  Since this was
the only user of the hook, this patch also removes the hook.

gdb/ChangeLog:

	* defs.h (deprecated_register_changed_hook): Remove prototype.
	* interps.c (clear_iterpreter_hooks): Remove reference to
	deprecated_register_changed_hook.
	* top.c (deprecated_register_changed_hook): Remove prototype.
	* valops.c (value_assign): Remove reference to
	deprecated_register_changed_hook.
	* tui/tui-hooks.c (tui_register_changed): Add parameter "frame".
	Add comment documenting the function.
	(tui_register_changed_observer): Define.
	(tui_install_hooks): Remove reference to
	deprecated_register_changed_hook.  Set
	tui_register_changed_observer.
	(tui_remove_hooks): Remove reference to
	deprecated_register_changed_hook.  Unset
	tui_register_changed_observer.