Separate out data item window

Separate out data item window

This introduces a new subclass of tui_gen_win_info for the data item
windows, letting us remove another element from tui_which_element.

gdb/ChangeLog
2019-06-25  Tom Tromey  <tom@tromey.com>

	* tui/tui-regs.c (tui_show_registers, tui_show_register_group)
	(tui_display_registers_from, tui_check_register_values): Update.
	(tui_display_register): Remove win_info parameter; update.
	(tui_get_register): Change type of parameters.
	* tui/tui-data.h (struct tui_data_element): Remove.
	(union tui_which_element) <data>: Remove.
	<data_window>: Change type.
	(struct tui_data_item_window): New.
	* tui/tui-data.c (init_content_element): Remove DATA_ITEM_WIN
	case.  Add assert.
	(~tui_data_item_window): New destructor.
	(free_content_elements): Remove DATA_ITEM_WIN case.