Genetic variants are not added up together when colouring with two UniProts the same element

related to #1815.

When one element is colored by two UniProt Ids - the sum of variants should be displayed in the browser. Currently we see only variants for one UniProt Id.

Also, the overlay bar is displayed two times - is it to show how each UniProtID colors the element? Maybe we should leave one bar with "color of a sum", @marek.ostaszewski what do you think?