Make distinction between STX "wallet balance" and available balance based on pending transfers/fees/swaps

[314159265359879]

This is confusing for users "Available balance is 0.03" but I can't send 0.028 STX...

This is what is really going on

• There is a balance of 0.03 in the wallet
• there is a pending transaction for 0.002 + 0.003 STX fee
  the "available balance = 0.025 STX" because it should be calculated as available in the wallet minus STX transfers out minus fees pending. And transactions with postconditions "transfer exactly x STX out" could also be subtraced from the balance to arrive at the most accurate available balance (for example a swap transaction STX --> xBTC on alexlab.co).

The current wallet version 4.6.0 will let me send 0.027 with a 0.003 STX fee which is just as impossible (available is 0.025). It does not yet take into account fees, it should.

The pending transaction here will remain pending until it is dropped.

Related to solutions for: #3338

[314159265359879]

I believe this is going to help resolve this issue: #3178

[markmhendrickson]

This may be very tricky to handle purely on the client-side. We may need new API functionality that helps calculate the effective available balance we should use in the wallet.

We could push this idea more and be even more conservative in cases in which a contract execution is pending in the mempool with a post condition for transferring an asset "up to" a particular amount (not just "exactly" an amount). The API could possibly provide this second, more conservative "worst case available" amount that the wallet could use to display a warning, but not block (e.g. "You have transactions pending that could result in spending more assets than you have available to make this one. Are you sure you want to proceed?").

@314159265359879 Mind creating a relevant issue in the API repo to help push this along?