Add ability to reorder legend elements in unified legend

[jett-crowdis]

Currently, the unified legend uses default matplotlib ordering, and it is very annoying to reorder the plot elements. It should be relatively straightforward to allow the passing of a list that will reorder legend elements within a set of legend elements.

[bjpop]

I second this. Currently I have one legend out of 7 where the order is unwanted: "1 3 2". I would like to be able to change to "1 2 3" more easily. Also, I found the documentation on how to fix this hard to understand.