Change tui_data_item_window::content to be a unique_xmalloc_ptr

Change tui_data_item_window::content to be a unique_xmalloc_ptr

This changes tui_data_item_window::content to be a unique_xmalloc_ptr
and fixes up the fallout.  It also removes a parameter from
tui_expand_tabs, as it was only ever given one value.

This also removes some tab-handling code from
tui_data_window::display_registers_from.  Because the content can only
be set by tui_register_format, and because that calls tui_expand_tabs,
it's not possible to see a tab here.

gdb/ChangeLog
2019-08-20  Tom Tromey  <tom@tromey.com>

	* tui/tui-regs.h (struct tui_data_item_window)
	<~tui_data_item_window>: Remove.
	<content>: Now a unique_xmalloc_ptr.
	* tui/tui-regs.c (tui_register_format): Return a
	unique_xmalloc_ptr.
	(tui_get_register): Update.
	(~tui_data_item_window): Remove.
	(tui_data_window::display_registers_from, tui_display_register):
	Update.
	* tui/tui-io.h (tui_expand_tabs): Update.
	* tui/tui-io.c (tui_expand_tabs): Return a unique_xmalloc_ptr.
	Remove "col" parameter.