studio: When loading a transaction, we want to be able to view the root transaction

[epost]

Reproduce:

1) Load a transaction by entering its hash in the studio. 2) Wait for its previous transactions to load. 3) The root transaction should now appear as a 'ghost' (i.e. unloaded) transaction. (Reason: the hash appears in the cache but its body isn't actually loaded, because that is explicitly prevented in the code, I think.)

Because the root transaction is a ghost, clicking on it doesn't show its info.

Update: This may have been due to some local WIP code in my workdir, esp:

getPrevious :: TxSum -> Maybe TxId
-- getPrevious = evalTxSum (const Nothing) (Just <<< _.previous) (Just <<< _.previous) (Just <<< _.previous)
getPrevious = evalTxSum (const Nothing) (const Nothing) (Just <<< _.previous) (Just <<< _.previous)