Change members of tui_locator_window to std::string

Change members of tui_locator_window to std::string

This changes two members of tui_locator_window to have type
std::string.  This removes a static limit.

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

	* tui/tui-stack.h (MAX_LOCATOR_ELEMENT_LEN): Remove define.
	(struct tui_locator_window) <full_name, proc_name>: Now
	std::string.
	* tui/tui-stack.c (tui_locator_window::make_status_line)
	(tui_locator_window::set_locator_fullname)
	(tui_locator_window::set_locator_info): Update.
	* tui/tui-source.c (tui_source_window::set_contents)
	(tui_source_window::showing_source_p): Update.