Move "fullname" to tui_source_window

Move "fullname" to tui_source_window

The "fullname" field in tui_source_window_base is only used by one
subclass.  This patch moves the field to that subclass, and changes it
to be a unique_xmalloc_ptr.

gdb/ChangeLog
2019-09-20  Tom Tromey  <tom@tromey.com>

	* tui/tui-winsource.h (struct tui_source_window_base)
	<~tui_source_window_base>: Don't declare.
	<fullname>: Remove.
	* tui/tui-winsource.c (~tui_source_window_base): Remove.
	* tui/tui-source.h (struct tui_source_window) <fullname>: New
	member.
	* tui/tui-source.c (tui_source_window::set_contents): Update.
	(tui_source_window::location_matches_p)
	(tui_source_window::maybe_update): Update.