combine_elements should work by property name, not position

This is a one line fix that makes sure combine_elements implements inheritance by property name rather than position. This allows child theme elements to have additional properties that the parent elements don't. 

For example, I made a plot where text axis labels have graphical icons attached, by implementing a class that inherited from `element_text`, with additional properties and an `element_grob` method. This worked but required this small change.