fix(GlButton): Update disabled button icon fill color

What does this MR do?

Closes #1047 (closed). Updates the color of the icon in a disabled button. Currently, it's almost indistinguishable from a normal icon in a button, since the fill color actually doesn't change when disabled. It creates confusion as to whether it's actually disabled. Was a finding in gitlab!46205 (comment 437007898).

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Closes #1047 (closed)