This fixes a bug that, when the supplied assets change the AssetSelector resets its
selected asset as well
Previously, it would keep the selected asset around, even when the asset list has changed,
leaving it in an invalid state
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