Introduce tui_gen_win_info::name method

Introduce tui_gen_win_info::name method

This removes tui_win_name and replaces it with a method on
tui_gen_win_info, removing another spot that switched on window type.

gdb/ChangeLog
2019-06-25  Tom Tromey  <tom@tromey.com>

	* tui/tui-win.c (window_name_completer, tui_set_focus)
	(tui_all_windows_info): Use name method.
	* tui/tui-data.h (struct tui_gen_win_info)
	(struct tui_source_window, struct tui_disasm_window)
	(struct tui_data_window, struct tui_cmd_window) <name>: New
	method.
	(tui_win_name): Don't declare.
	* tui/tui-data.c (tui_partial_win_by_name): Use name method.
	(tui_win_name): Remove.