Introduce enum tui_box

Introduce enum tui_box

This adds a new enum to the TUI, replacing two #define constants,
providing better type safety.

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

	* tui/tui-wingeneral.h (tui_make_window): Change type of "box_it"
	parameter.
	* tui/tui-wingeneral.c (tui_make_window): Change type of "box_it"
	parameter.
	(tui_gen_win_info::make_visible): Update.
	* tui/tui-layout.c (init_and_make_win): Change type of "box_it"
	parameter.
	* tui/tui-data.h (enum tui_box): New enum.
	(BOX_WINDOW, DONT_BOX_WINDOW): Remove defines.