Add a radio button to SEND tab for spending "shuffled" or "unshuffled" coins, and a grayed banner (with advisory text) for when shuffle is off for the wallet

[cculianu]

The radio button would select between two possible modes of operation:

• Spending shuffled coins (only!) together with change going to a unique new change address
• Spending unshuffled coins (only!) together with change going to an address not previously involved in any shuffle-spending.

The above is only when cashshuffle is enabled. A disabled cashshuffle wallet behaves as before -- with the exception that if the plugin is loaded and in memory (that is, wasn't explicitly unloaded by the user) -- it will still show a grayed banner in the send tab telling them what they are doing and the privacy implications of this.

Users must unload the plugin for the banners to disappear altogether.

[cculianu]

So overall there will be 4 states to the app:

1. Plugin is loaded and enabled for this wallet + send tx's are shuffled-only (segregated)
2. Plugin is loaded and enabled for this wallet + send tx's are unshuffled-only (segregated)
3. Plugin is loaded but NOT enabled for this wallet (send tx's can be mixed) -- add a gray banner here warning user -- this mode's spending behavior is identical to old non-shuffle EC.
4. Plugin is not loaded at all (code is not even in memory!) -- UI looks more-or-less just like regular EC. No warning banner. No privacy protection whatsoever.

[cculianu]

This issue now deprecates #28.