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!)

![Screen_Shot_2016-04-28_at_10.35.04](/uploads/41f45f3f92f971e7f38df7a98765cb9d/Screen_Shot_2016-04-28_at_10.35.04.png)

Closes #16555, #14820

See merge request !3963