Move TUI data item window to tui-regs.h

Move TUI data item window to tui-regs.h

The TUI data item window is only used by the TUI register window.  So,
this patch moves the relevant code to tui-regs.[ch].

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

	* tui/tui-regs.h (struct tui_data_item_window): Move from
	tui-data.h.
	* tui/tui-regs.c (tui_data_item_window): Move from tui-data.c.
	* tui/tui-data.h (struct tui_data_item_window): Move to
	tui-regs.h.
	* tui/tui-data.c (~tui_data_item_window): Move to tui-regs.c.