BIP329: Add optional key origin property and expand truncation note #1412

Based on feedback received after BIP329 was initially published, this adds the following:

• An optional origin property containing key origin information which can be used to to disambiguate labels from different wallets contained in the same export
• A note on warning users if truncation is applied, along with a suggested maximum label length

On further consideration, the origin property has been changed to reflect an abbreviated output descriptor (including key origin information but excluding actual keys, child path elements or a checksum). This is necessary to handle multiple script types and more complex wallets including multisig wallets.