Remove can_highlight from TUI windows

Remove can_highlight from TUI windows

Each TUI window has a "can_highlight" member.  However, this has the
same meaning as "can_box" -- a window can be highlighted if and only
if it can be boxed.  So, this patch removes can_highlight in favor of
simply using can_box.

gdb/ChangeLog
2019-11-10  Tom Tromey  <tom@tromey.com>

	* tui/tui-wingeneral.c (tui_unhighlight_win): Use can_box.
	(tui_highlight_win): Likewise.
	(tui_win_info::check_and_display_highlight_if_needed): Likewise.
	* tui/tui-data.h (struct tui_win_info) <can_highlight>: Remove.
	* tui/tui-command.h (struct tui_cmd_window) <tui_cmd_window>:
	Don't set can_highlight.

Change-Id: I35916859070efcdfcc6e692c71cc6070956dcfce