Improve tx input review

[ImplOfAnImpl]

In general, inputs that are commands have to be reviewed by the user before the tx is signed.

The blockchain has several kinds of "input commands":

• account commands (AccountCommand or OrderAccountCommand); only one of these is allowed per tx;
• account spendings (AccountSpending), which currently only allow spending from a delegation; any number of these is allowed per tx and they can co-exist with an account command.

Currently, the app assumes that only one "input command" is possible per tx. Worse, it silently overwrites the last stored "input command" when a new one arrives. So

• At the very least, the app should reject txs with more than one "input command" (this is not a huge deal, because it's unlikely that anyone will create txs like this).
  Addressed by Fixing issues #23
• Ideally, present all "input commands" for review one by one, same as it's done for outputs.

There are a couple of TODOs related to this, the main one is near TxSummaryCollector::input_command.