When tokens are added/removed, the asset list in the AssetAndAmountInput is updated.
The selected asset can be specified by the parent component which is needed for things like sticker market where we need to set SNT as the fixed token.
Improved the validation for the component:
- validate() can be called externally
- validation display is handled internally and messages can be customised
- validation error messages are handled by the Input component and validation UX is consistent with other tx components