Introduce set_highlight method

Introduce set_highlight method

This introduces the tui_win_info::set_highlight method, and changes
the highlighting-related code to use bool rather than int.

gdb/ChangeLog
2019-06-25  Tom Tromey  <tom@tromey.com>

	* tui/tui-wingeneral.c (tui_unhighlight_win, tui_highlight_win):
	Update.
	* tui/tui-layout.c (make_command_window)
	(show_source_disasm_command, show_data, init_and_make_win)
	(show_source_or_disasm_and_command): Update.
	* tui/tui-data.h (struct tui_win_info) <set_highlight>: New
	method.
	<can_highight, is_highlighted>: Now bool.
	(tui_set_win_highlight): Don't declare.
	* tui/tui-data.c (tui_set_win_highlight): Remove.