Change TUI source window iteration

Change TUI source window iteration

Currently the TUI does separate bookkeeping to track which source
windows exist.  It seems better to me to just refer to the list of
windows for this, so this patch removes the special handling and
instead adds a new iterator.

gdb/ChangeLog
2019-08-15  Tom Tromey  <tom@tromey.com>

	* tui/tui-winsource.h (struct tui_source_window_iterator): New.
	(struct tui_source_windows): New.
	* tui/tui-winsource.c (tui_display_main): Update.
	* tui/tui-win.c (tui_resize_all, tui_adjust_win_heights)
	(new_height_ok, parse_scrolling_args): Update.
	* tui/tui-layout.c (show_layout, show_data): Update.
	* tui/tui-data.h (tui_source_windows, tui_clear_source_windows)
	(tui_add_to_source_windows): Don't declare.
	* tui/tui-data.c (source_windows, tui_source_windows)
	(tui_clear_source_windows, tui_add_to_source_windows): Remove.