workflows: workflow representation fix

* Calculates the path of tasks that workflow has
  followed and represents with different color
  the completed, last and blocked tasks.

Signed-off-by: Georgios Papoutsakis <georgios.papoutsakis@cern.ch>
workflows: workflow representation improvements

Signed-off-by: Georgios Papoutsakis <georgios.papoutsakis@cern.ch>