multi-level family expand/collapse in the dot view

[hjoliver]

cylc/cylc-flow#1881 added family expand/collapse to the dot view, but only for top level families. To be consistent with the other views we should be able to click down through intermediate family levels too, if there are any.

@oliver-sanders - assigning you as this might possibly be an easy extension to your original change (if we're lucky 😬).