SPIKE: Swap improvements

[aristidesstaffieri]

It's possible to swap for the same asset, in which case users will just lose money with the fees.
Should we prevent swapping to the same asset?
As an alternative, should we display a warning to make it clear to users they're swapping for the same asset and will lose funds with the fee charged?

Users also can't "flip" the assets being swapped, which is a common feature of swap UIs in other wallets. Should we implement it?