Use macros for TUI window names

Use macros for TUI window names

Christian pointed out that tui-layout.c hard-codes various window
names.  This patch changes the code to use the macros from tui-data.h
instead.  For each window, I searched for uses of the name; but I only
found any in tui-layout.c.  This also adds a new macro to account for
the "status" window.

gdb/ChangeLog
2020-06-16  Tom Tromey  <tom@tromey.com>

	* tui/tui-data.h (STATUS_NAME): New macro.
	* tui/tui-layout.c (tui_remove_some_windows)
	(initialize_known_windows, tui_register_window)
	(tui_layout_split::remove_windows, initialize_layouts)
	(tui_new_layout_command): Don't use hard-coded window names.