Feature request: Allow coin control for post-mix coins

Created by: hkjn

(This is not a bug as much as a feature request / suggestion for UX improvement. Feel free to close or merge into other issues if this is known and on the roadmap, and thanks for the great work to date!)

With whirlpool-gui + CLI version 0.7.0 and Samourai Android version 0.99.74, I found it very difficult to create a one input / one output tx spending from the post-CJ coins.

What I'd like to do is to take a single post-Whirlpool mix coin as input, and send to an address of my choice, with configurable miner fee deducted from the single output. E.g with a 5 000 000 sat output, if I sent to an address generated from a cold storage wallet and wasn't in a hurry, I'd expect that a 110 byte transaction paying 173 sats (1 sat / vbyte) would be possible.

With the current Samourai Android wallet, trying to create such a transaction would require guessing what amount to specify to only consume one output, and when I tried this, I didn't succeed, and the resulting transaction consumed two 5M sat outputs instead of one, sending one back as change.

Since the whirlpool-gui already shows individual coins (using the term "utxo"), it would be great if the mobile wallet gave the same level of control. Alternatively / additionally, maybe the "Postmix" view in the desktop GUI also could allow spending that coin, and not just entering it into further mixes.