Implement title_format

This implements the title_format command, with a new placeholder %shell
which gets substituted with the view type (xwayland, xdg_shell_v6 or
wl_shell).

Example config:

    for_window [title=".*"] title_format %title (class=%class instance=%instance shell=%shell)

sway/commands.c

@@ -182,6 +182,7 @@ static struct cmd_handler command_handlers[] = {
 	{ "splith", cmd_splith },
 	{ "splitt", cmd_splitt },
 	{ "splitv", cmd_splitv },
+	{ "title_format", cmd_title_format },
 };
 
 static int handler_compare(const void *_a, const void *_b) {