Feature request: Add the ability to swap currency side

Some transactions require me to use two currency in one posting. It would be nice if cone can have the ability to swap the currency side while entering transaction.

Example transaction:

```
2023-02-03 Sell
assets:bank a:saving Rp300000
assets:bank b:saving -30 USD
```

Thanks!

Platform: Android 13
Cone version: 0.2.20

Start at Cone's transaction-entry flow and trace how the two currency amounts in the example are entered and assigned to posting sides. Define the interaction for swapping sides, then verify that the example transaction can be entered with either currency side selected and that the resulting posting remains correct.

Written by the indexing model from the issue text.