workflow: support multi workflow views

[oliver-sanders]

Snippet taken from #87:

We should aim to make views multi-workflow compatible so that we have the potential to open multiple workflows in a single view.

The design sketch shows this working via multi-selection in GScan, however, it would be easier / more intuative to start off by adding this to the "workflow-part" node (i.e. foo/ in the above sketch).

#1108 has moved this one step closer, here's a screenshot showing this functionality working by accident:

To implement this for real (post #1108) we would need to:

• Issue the "workflow" subscription for the "workflow-part" (e.g. foo/*).
• Make the "workflow-part" nodes in GScan linkable.
• Update the housekeeping to handle multi workflow use.
• Update the workflow toolbar, namely the play/pause/stop buttons.

Pull requests welcome!