workflows: make workflow directories interactive

[oliver-sanders]

Currently, each workflow in the workflows view has a state icon. Clicking on this icon brings up the mutation menu.

To make it easier to manage groups of workflows, we should provide an icon for workflow directories too. This would allow the user to operate on a directory of workflows as a group as they might do on the command line.

E.G. For this hierarchy:

• foo
  • run1
  • run2
  • run3

Each of the runs will have an icon, but at present, the directory foo does not.

Suggestions:

• Use the burger-bar-like button that we currently use in the workspace toolbar for a workflow directory.
• In the mutation menu, display it as a "workflow group" and state the number of workflows contained in that group.

Note, we will likely want to implement multiple selection one day so "group" might make more sense than "directory" or "hierarchy" in this context.

This should be relatively simple, mutations should be provided with a workflow ID glob e.g. foo/*.

Note, Cylc commands should filter workflows by the appropriate state e.g. cylc clean foo/* will only remove stopped workflows.