ecash: add XEC option to the GUI

Summary:
Running bitcoin-qt with -xec will show XEC and sats, running without will show BCHA and sats.

Depends on D9672

Test Plan:

ninja all check
./src/qt/bitcoin-qt -regtest -xec=[0|1]

make sure the amounts all look correct when switching between units

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D9673