Introduce TUI window iterator

Introduce TUI window iterator

This introduces an iterator class and a range adapter to make it
simpler to iterate over TUI windows.

One explicit iteration remains, in tui-win.c, because that spot is
deleting windows as well.

gdb/ChangeLog
2019-07-17  Tom Tromey  <tom@tromey.com>

	* tui/tui-wingeneral.h (tui_refresh_all): Update.
	* tui/tui-wingeneral.c (make_all_visible): Use foreach.
	(tui_refresh_all): Remove "list" parameter.  Use foreach.
	* tui/tui-win.c (window_name_completer): Use foreach.
	(tui_refresh_all_win, tui_rehighlight_all, tui_all_windows_info)
	(update_tab_width): Likewise.
	* tui/tui-layout.c (show_layout): Update.
	* tui/tui-data.h (class tui_window_iterator): New.
	(struct all_tui_windows): New.
	* tui/tui-data.c (tui_partial_win_by_name): Use foreach.