Review `displayedWorkflowInfo` and `rootWorkflow` store

The app can now only display a single workflow: subworkflows are displayed in their own tab (i.e. different app). Therefore, there is no longer a rootWorkflow concept nor a displayedWorkflow concept.

We should then review the related store and remove duplicated info. For example, I guess that displayedWorkflowInfo.id and rootWorkflowId are always equal in the whole lifecycle of the app.

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