Decouple wallet from the chain layer

[ghost]

Why

• On-chain interaction is currently tied to our TinyWallet which is very restricted and somewhat complicated
• Signing and balancing transaction is actually a somewhat intricate process which we sidestepped by using a marked UTxO which has its own problem, the main one being that it needs to exist in the first place
• Users might not want to entrust yet another wallet with funds and secrets, or want to keep a centralised wallet, or use hardware wallets/key-vaults
• There is a specification CIP-30 for Dapp-Wallet interaction. It's currently very much geared towards browser-based JS integration but the general idea is that DApp should not hold secrets and delegate signatures

What

• As a first step towards enabling external wallet, turn out tiny wallet into an completely independent component
• Make Direct chain component communicates with this "external" wallet through a single signAndBalance function

[ch1bo]

Related / alternative to #215