Support CashFusion

[haplo]

It would be great for privacy if Paytaca wallet supported CashFusion to improve privacy.

Pokket wallet supports CashFusion, it should be possible to reuse some of its code as its MIT licensed.

For a good UX enabling/disabling CashFusion can be done through a setting, defaulting to disabled but with maybe with a display-once prompt on app start.

When CashFusion is enabled outstanding balance should display both total and also ready-to-spend.

The spend screen should have some logic to favor fused coins, and if the user tries to send more than the ready balance display a warning and have an override switch to send anyway.