[backport#16944][gui] send: include watch-only

Summary:
For wallets with WALLET_FLAG_DISABLE_PRIVATE_KEYS.

https://github.com/bitcoin/bitcoin/pull/16944/commits/848f88920853724511387ca0b7ef652fa14ced71

Depends on D7293

Partial backport of Core PR16944

Test Plan:

ninja check

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7294