Refactoring the Plutus playground client.

[merivale]

New guy moves in a couple of months ago, now he wants to redecorate...

This PR moves code around with a view to:

• making more room for the impending remodelling (SCP-1393, SCP-1562)
• bringing things closer to some conventions adopted in the Marlowe playground client code
• dividing things up according to (what strike me as) natural lines that have emerged since the previous structure was put in place
• making all exports explicit.

Doubts/questions:

• Is having separate Action.Lenses and Wallet.Lenses modules too much? Maybe the lenses there should all go together in Simulator.Lenses (which doesn't exist, but which might deserve to exist after the remodelling).
• Should it be Simulator.View or Simulation.View? I chose the former to match Editor, and because it's called the simulator on the actual page. But its purpose is to visualise the simulations, and Simulation matches other things (Wallet, Action, Transaction).

[krisajenkins]

LGTM. Thank the lord that the git CLI has syntax highlighting for, "this line didn't change, it just moved," these days. That made it easy.