Remove separate visibility flag

Remove separate visibility flag

TUI windows keep track of their visibility in a boolean field.
However, this is not needed, because a window is visible if and only
if it has an underlying curses handle.  So, we can remove this
separate field.

gdb/ChangeLog
2019-08-16  Tom Tromey  <tom@tromey.com>

	* tui/tui.c (tui_is_window_visible): Update.
	* tui/tui-wingeneral.c (tui_make_window)
	(tui_gen_win_info::make_visible, tui_refresh_all): Update.
	* tui/tui-win.c (window_name_completer, tui_refresh_all_win)
	(tui_set_focus_command, tui_all_windows_info, update_tab_width)
	(tui_set_win_height_command, parse_scrolling_args): Update.
	* tui/tui-source.c (tui_source_window::style_changed): Update.
	* tui/tui-regs.c (tui_show_registers)
	(tui_data_window::first_data_item_displayed)
	(tui_data_window::delete_data_content_windows)
	(tui_check_register_values, tui_reg_command): Update.
	* tui/tui-disasm.c (tui_show_disassem): Update.
	* tui/tui-data.h (struct tui_gen_win_info) <is_visible>: New
	method.
	<is_visible>: Remove field.
	* tui/tui-data.c (tui_next_win, tui_prev_win)
	(tui_delete_invisible_windows): Update.