props: add container property to PROP_INFO

So we can mark a property should be inside a container.
Use this info in pw_stream to construct properties.

spa/include/spa/param/props.h

@@ -42,6 +42,7 @@ enum spa_prop_info {
 					  *  is of the type of the property, the second
 					  *  one is a string with a user readable label
 					  *  for the value. */
+	SPA_PROP_INFO_container,	/**< type of container if any */
 };
 
 /** predefined properties for SPA_TYPE_OBJECT_Props */