Removes duplicates from the label dropdown

It concats the duplicate labels into a single label in the dropdown with the color being a gradient of the differnet colors being concatted.

I liked your idea @jschatz1 of using a standard icon color. However, with this i've taken all the duplicated colors & made a little gradient with them so it still half resembles the label colors (this has been limited to 4 or it could get crazy!)

Closes #16555 (closed), #14820 (closed)