Make tui_win_info::name pure virtual

It seemed cleaner to me for tui_win_info::name to be pure virtual.
This meant adding a name method to the locator window; but this too
seems like an improvement.

gdb/ChangeLog
2020-07-01  Tom Tromey  <tom@tromey.com>

	* tui/tui-data.h (struct tui_win_info) <name>: Now pure virtual.
	* tui/tui-stack.h (struct tui_locator_window) <name>: New method.