Move some code out of tui-data.h

This moves some code out of tui-data.h, to more closely related
places.  Some unused forward declarations are also removed.

gdb/ChangeLog
2020-07-01  Tom Tromey  <tom@tromey.com>

	* tui/tui-stack.c (SINGLE_KEY): Move from tui-data.h
	* tui/tui-winsource.h (enum tui_line_or_address_kind)
	(struct tui_line_or_address): Move from tui-data.h.
	* tui/tui-win.c (DEFAULT_TAB_LEN): Move from tui-data.h.
	* tui/tui-data.h (DEFAULT_TAB_LEN): Move to tui-win.c.
	(tui_cmd_window, tui_source_window_base, tui_source_window)
	(tui_disasm_window): Don't declare.
	(enum tui_line_or_address_kind, struct tui_line_or_address): Move
	to tui-winsource.h.
	(SINGLE_KEY): Move to tui-stack.c.