Add support for multiple outputs (address+amount) via BIP 21

[Klakurka]

Relies on the completion of https://reviews.bitcoinabc.org/T3401 in Cashtab (and ideally in Electrum ABC as well).

Adding a 2nd output for PB donations would technically be possible with this.

[Klakurka]

ecash:xxx?amount=500.69&addr=ecash:yyy&amount=555&addr=666&addr=ecash:zzz&amount=6969&op_return_raw=ASDF123

[Klakurka]

Let's make this a configurable % (say, any number positive number), with a reasonable default.

If the amount to be sent in the "optional donation" is less than, say, 10 XEC then don't include it.

Of course if somebody sets their donation % to 0 then it will never include the extra output.

[Klakurka]

Let's go with 2% and allow integer options between 0 and 100. Parameter name will be donation-rate.

• A donation rate of 100 means that 50% of the payment will go to the donation address.
• If a price is denominated in fiat, any donation rates above 5% will change the fiat value we state in the dialog. For 0-4, leave the fiat value as it was set in the parameters.