Change tui_which_element::data_window to be a pointer

Change tui_which_element::data_window to be a pointer

A coming patch will add a constructor to tui_gen_win_info.  However,
because the tui_which_element union contains an object of this type,
first something must be done here in order to avoid having a union
with a member that has a constructor.  This patch changes this element
to be a pointer instead.

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

	* tui/tui-wingeneral.c (tui_refresh_win): Update.
	* tui/tui-windata.c (tui_first_data_item_displayed)
	(tui_delete_data_content_windows): Update.
	* tui/tui-win.c (tui_data_window::set_new_height): Update.
	* tui/tui-regs.c (tui_show_registers, tui_show_register_group)
	(tui_display_registers_from, tui_check_register_values): Update.
	* tui/tui-data.h (union tui_which_element) <data_window>: Now a
	pointer.
	* tui/tui-data.c (init_content_element): Update.  Allocate the new
	window.
	(tui_free_data_content): Update.
	(free_content_elements) <case DATA_WIN>: Free the window.