Add checks to check for vertex / edge existence, highlighting etc.

[Willem3141]

Requested by Wouter: a check to test if a certain edge (between two labeled vertices) is highlighted. Note that this cannot be done with the Match highlighting check, as that one checks for isomorphism as well.

Similarly, we should have simple checks that check if a vertex with a certain label exists, if an edge between two labeled vertices exists, and so on.