Additional query filters for rem metadata

Currently, the only filter that has to do with the qualities of the rem rather than its connections or content is Rem Type. Additional possible filters: date created, date modified, ease, has source (or a pin reference), has image/video/audio/pdf, times practiced, times failed, success rate, SRS scheduler used, next practice date, last practice date. Some of these will obviously have to take numbers, so additional operators (more, less, equal) would also be required (may as well add number ranges if adding more and less).

Most directly useful for finding flashcards that are below the desired percentage/amount of correct answers, but I'm sure will also prove useful for all sorts of other things.