Add feature that the last subsplit can have a different icon than the mainsplit

[Xenoras]

Hey folks,

I was working on making my layout more appealing today when I encountered a problem. I have a split with 3 subsplits. With the usual way how to make subsplits the last subsplit has the same icon as the main split. However, I was looking for a way to have a different icon for the main split. Below is a picture of what I am trying to do. First one is how it is possible, second how i worked around it and third how it should be (the last one is photoshopped). I'd aprreciate if you could add this feature.

[TheTaraStark]

Can this be merged into #2131 since it seems like that'll be the solution that wraps these all neatly?