Change tui_make_status_line to be a method

Change tui_make_status_line to be a method

This changes tui_make_status_line to be a method on
tui_locator_window.  This is a minor cleanup.

This also changes the new method to use the locator's width, rather
than the terminal width.  This is important if we ever want to allow
windows to be made more narrow.

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

	* tui/tui-stack.h (struct tui_locator_window) <make_status_line>:
	Declare.
	* tui/tui-stack.c (tui_locator_window::make_status_line): Rename
	from tui_make_status_line.
	(tui_locator_window::rerender): Update.