Use bool for visibility

Use bool for visibility

This changes the visibility-related functions and data members in the
TUI to use bool rather than int.

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

	* tui/tui.h (tui_is_window_visible): Return bool.
	* tui/tui.c (tui_is_window_visible): Return bool.
	* tui/tui-wingeneral.c (tui_make_window, make_visible)
	(tui_make_visible, tui_make_invisible)
	(tui_win_info::make_visible)
	(tui_source_window_base::make_visible, make_all_visible)
	(tui_make_all_visible, tui_make_all_invisible): Update.
	* tui/tui-windata.c (tui_delete_data_content_windows): Update.
	* tui/tui-data.h (struct tui_gen_win_info) <is_visible>: Now
	bool.
	(struct tui_win_info, struct tui_source_window_base)
	(struct tui_cmd_window) <make_visible>: Change parameter to bool.
	* tui/tui-data.c (tui_init_generic_part): Update.