mash: Shelf doesn't coalesce multiple windows from the same mojo app.

This is a placeholder bug; I don't know if it manifests at the moment.
If a mojo app (eg. QuickLaunch) opens 2+ windows, they get separate shelf items.
Instead, it should coalesce windows under the same shelf item, like extensions.

One way to do this might be for the window server to assign ShelfIDs by client.
This is related to (blocking)  Issue 557406 , and (not blocking?)  Issue 681072 .