Fix mismatching chevron color in split dropdown buttons

What does this MR do?

Fixes the split dropdown chevron color by utilizing an SVG mask over an element whose contents are a box in the color of border-color. This was tested in Firefox, Edge, Chrome, and Safari.

before	after

The caveat is that this method does not work in Internet Explorer and some pre-Blink versions of Edge:

Edge 17	Edge 18

I am not sure whether we technically support Edge 17 anymore, do we @leipert?

Does this MR meet the acceptance criteria?

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Security

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Closes #1040 (closed)