Werk #3148: fixed Edit View/Dashlet Dialogs offering non-sensical filter choices

This is actually two separate but similar bugfixes: When editing a dashlet that represented a view, the search/context filter offered all possible filter choices even when they made no sense for the data visualised. Also, when editing a aggregation group view (and potentially other views), the filter list offered the same filter option (aggregation group) twice, with different semantics, causing exceptions if either one of these options was used.

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