Only restore workflow when coming back from Monitor page

Having taken a step back in !338 (merged), I could reuse my workflow id store to reimplement the restoring feature but without going beyond the scope.

I know it is a quick fix and not ideal and that we should wait to sanitize the graph/workflow slices. However, this feature is saving me a lot of time and clicks when working on a workflow so it is important for me to have it.

Also, the reduced scope means it no longer interferes with other opening mechanisms and that it will be easier to remove/change when needed.

The store was renamed in a separate commit (e0582656) to reflect its actual purpose.