use XEC or BCH for bitcoin-qt depending on CLI setting

Summary:

• removed rarely used units in favor of a simpler base/subunit scheme
• had to do some type juggling in the BitcoinUnits::factor(int) function to replace the magic numbers with the Currency class
• fixed the bitcoin-qt tests to account for XEC's decimal places

Test Plan:

ninja all check check-functional
./src/qt/bitcoin-qt -regtest

see that amounts are formatted correctly

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D9672