Add a menu for Dask cluster setup and configuration in the HistomicsTK front-end

[cdeepakroy]

Here is some info on how to setup a Dask cluster

Once the cluster is setup, the cluster scheduler address can then be passed to Slicer CLIs of HistomicsTK that work on whole-slide images.

[cdeepakroy]

CC: @jbeezley @manthey @cooperlab