xtriggers: use foreign task id for workflow_state xtriggers

[oliver-sanders]

workflow_state xtriggers allow us to trigger off of outputs in another workflow:

[scheduling]
    initial cycle point = 2010
    [[xtriggers]]
         upstream = workflow_state(workflow=workflow_id, task=foo, point=%(point)s, \
            message='data ready'):PT10S
   [[graph]]
        P1Y = """
            @upstream => whatever
        """

The data store maintains a list of graph edges, these edges have IDs composed of the tasks they involve e.g. 1/@trig|1/b.

At the moment the edge IDs for workflow_state xtriggers reference the xtrigger rather than the task the xtrigger is pointing at.

I think we should change this so the xtrigger acts as an alias pointing at the foreign task using it's absolute ID (e.g. ~user/workflow//cycle/task). This would allow the UI to follow edges from one workflow to another allowing us to graph a collection of inter-dependent workflows.

Note: Would need to ensure that egde IDs can are safely parsed whether relative to the current workflow (as at the moment) or absolute (as this change would introduce the potential for) in both cylc-flow and cylc-ui.

Note: Edge IDs must be unique to enter in the store so it might be necessary to check for duplication in xtrigger definitions for safety.

Additional context

Pull requests welcome!