Change tui_make_window to be a method

Change tui_make_window to be a method

I combined several small changes into one patch here.  I believe I
started by noticing that the "title" is not needed by tui_gen_win_info
and could be self-managing (i.e. std::string).  Moving this revealed
that "can_box" is also a property of tui_win_info and not
tui_gen_win_info; and this in turn caused the changes to
tui_make_window and box_win.

2019-08-20  Tom Tromey  <tom@tromey.com>

	* tui/tui-wingeneral.h (tui_make_window): Don't declare.
	* tui/tui-wingeneral.c (box_win): Change type of win_info.
	(box_win): Update.
	(tui_gen_win_info::make_window): Rename from tui_make_window.
	(tui_win_info::make_window): New method.
	(tui_gen_win_info::make_visible): Update.
	* tui/tui-source.c (tui_source_window::set_contents): Update.
	* tui/tui-regs.c (tui_data_window::show_register_group): Update.
	(tui_data_window::display_registers_from): Update.
	* tui/tui-layout.c (tui_gen_win_info::resize): Update.
	* tui/tui-data.h (struct tui_gen_win_info) <make_window>:
	Declare.
	<can_box>: Remove.
	<title>: Remove.
	(struct tui_win_info) <make_window>: Declare.
	<can_box>: Now virtual.
	<title>: New member.
	* tui/tui-data.c (~tui_gen_win_info): Don't free title.
	* tui/tui-command.c (tui_cmd_window::resize): Update.