Derive tui_locator_window from tui_win_info

tui_locator_window is the last remaining concrete child class of
tui_gen_win_info.  It seems a bit cleaner to me to flatten the
hierarchy a bit; this patch prepares for that by changing
tui_locator_window to derive from tui_win_info.

gdb/ChangeLog
2020-07-01  Tom Tromey  <tom@tromey.com>

	* tui/tui-stack.h (struct tui_locator_window): Derive from
	tui_win_info.
	<do_scroll_horizontal, do_scroll_vertical>: New methods.
	<can_box>: New method.