MDCList performance patch for listElements()

[erestor]

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• Added element cache to MDCList listElements getter

This fixes performance of complexity O(n^2) operations which call listElements() within a loop. See MDCListFoundation.getSelectedIndexFromDOM which becomes noticeably slow once a list has several hundred items (e.g. a selection of countries) and unusable for several thousand items, forcing clients to implement measures against long lists which might not be feasible for them.

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