Comment 13 for bug 927898

When the user toggles one slave toggle, the master should not be affected and keep its previous state.
As you mentioned, there is a degenerate case when the master is on but no slave is selected. In this case toggling the master on or off would be useless. IMHO, this could be acceptable in a first stage of development because if the user toggles all slaves off, it's his choice and he should know what he has done (anyway better than not letting him turn all of them off). Of course, a more elaborate solution could be to turn off and gray out the master, to indicate that it has become useless (but still let right-click on it to open its slaves selection dialog; I suppose you don't like this right-click solution because you mentioned the properties dialog only: actually my idea is based on this quick access because without it, too few buttons would reduce usability a lot).

Anyway, I don't dislike the 3-4 user configurable snapping configurations buttons exposing the vector of all sources and targets. I think it's a good candidate to start trying to expose all of them and see if users like (and can take advantage of) a such detailed feature.