Don't show duplicate picker sets in proofreading toolbox

[srjfoo]

If there are character suites equipped for a project that contain identical picker sets, the duplicates stack (see screenshot).

Currently, this only affects the Greek picker sets, because Basic Greek is a proper subset of Polytonic Greek, and the picker set in Basic Greek is identical to the first picker set in Polytonic Greek.

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Historical note: We have both Basic Greek and Polytonic Greek because there are occasions where only the basic alphabet is needed, and so it seemed excessive to force the PM to have a large number of unnecessary picker sets for those projects. We do not currently have any other character suites where there are identical picker sets.

Position of characters in the picker sets does not matter. If exactly the same characters are in two picker sets, they will stack in the toolbox.