dashboard: Trying to delete dataset subtree shouldn't silently do nothing

[dnadlinger]

On current master (16b917b, Qt 5.6.2), trying to delete an entire subtree in the Datasets dock using the context menu silently does nothing.

While I'm not sure whether it makes sense to support that, it should either do what is expected (delete all the child keys, possibly after a prompt), or not be offered as an option.