window-rules: support title criteria

Example config:

    <windowRules>
      <windowRule identifier="foot" title="max">
        <action name="Maximize"/>
      </windowRule>
    </windowRules>

Observe that:

- `foot -T foo` starts maximized
- `xterm -T foo` starts normal

include/window-rules.h

@@ -20,6 +20,7 @@ enum property {
  */
 struct window_rule {
 	char *identifier;
+	char *title;
 
 	enum window_rule_event event;
 	struct wl_list actions;