restrict random button to subsets of entries

[De7vID]

Request is to allow "Random" button to be configured so that it returns only entries from, e.g., KLCP1 or TKD.

Basically blocked by #406.

[LievenLitaer]

I think there are things that are more important to fix. Adding the KLCP tags is a lot of work (well, 500 entries), and since the KLCP is not a canon source (i.e. not a fact), which may be altered one day, having this may cause lots of problems later.