menu: support <action name="Execute"> option <execute>

<exectue> is a deprecated name for <command>.
See: http://openbox.org/wiki/Help:Actions#Action_syntax

But some openbox3 menu generators still use it, for example
https://wiki.archlinux.org/title/xdg-menu - so let's support it for
backward compatibility.

docs/labwc-actions.5.scd

@@ -12,7 +12,9 @@ Actions are used in keyboard bindings.
 	Close top-most view.
 
 *<action name="Execute"><command>*
-	Execute command.
+	Execute command.  Note that in the interest of backward compatibility,
+	labwc supports <execute> as an alternative to <command> even though
+	openbox documentation states that it is deprecated.
 
 *<action name="Exit">*
 	Exit labwc.

src/menu/menu.c

@@ -180,6 +180,13 @@ fill_item(char *nodename, char *content)
 			"nodename: '%s' content: '%s'", nodename, content);
 	} else if (!strcmp(nodename, "command.action")) {
 		current_item_action->arg = strdup(content);
+	} else if (!strcmp(nodename, "execute.action")) {
+		/*
+		 * <action name="Execute"><execute>foo</execute></action>
+		 * is deprecated, but we support it anyway for backward
+		 * compatibility with old openbox-menu generators
+		 */
+		current_item_action->arg = strdup(content);
 	}
 }