Change TUI window iteration

Change TUI window iteration

This changes the TUI to track all the instantiated windows in a new
global vector.  After this, iteration over TUI windows is done by
simply iterating over this vector.

This approach makes it simpler to define new window types.  In
particular, a subsequent patch will add the ability to define a TUI
window from Python.

Note that this series will not remove tui_win_list.  This will
continue to exist in parallel, only because it was simpler to leave
this alone.  Perhaps it could still be removed in the future.

gdb/ChangeLog
2020-02-22  Tom Tromey  <tom@tromey.com>

	* tui/tui-winsource.h (struct tui_source_window_iterator)
	<inner_iterator>: New etytypedef.
	<tui_source_window_iterator>: Take "end" parameter.
	<tui_source_window_iterator>: Take iterator.
	<operator*, advance>: Update.
	<m_iter>: Change type.
	<m_end>: New field.
	(struct tui_source_windows) <begin, end>: Update.
	* tui/tui-layout.c (tui_windows): New global.
	(tui_apply_current_layout): Clear tui_windows.
	(tui_layout_window::apply): Update tui_windows.
	* tui/tui-data.h (tui_windows): Declare.
	(all_tui_windows): Now inline function.
	(class tui_window_iterator, struct all_tui_windows): Remove.

Change-Id: I6ab77976d6326f427178f725434f8f82046e0bbf