Refactor PSBT signers UI

[dr-orlovsky]

A signer is a specific account-level xpub, not a device. This, PSBT UI should list a specific signers and not devices - indicating how many signatures satisfying different spending conditions a signer can produce (usefult for taproot, where the same key can sign multiple times).

Signers coming from the same device but having different accounts (i.e. most likely used in different spending conditions) should be shown separately.