Highlight empty/non-empty relations in results selector

[hvitved]

For normal queries this is not super relevant, but when you run a consistency query with many query predicates, it would be nice if e.g. empty relations use a grey font colour, or similar, so it is easy to spot which relations to check:

[adityasharad]

Excellent idea. cc @nuthinking who's been thinking about the display of results and result sets.

[nuthinking]

@adityasharad If the suggestion is to dim/disable some items because there is no point in using them, sure. But, just so you know, I'm not very knowledgeable on syntax writing :)