qt: change BCH to XEC in QLabel's default text

Summary: I noticed while backporting core#16432 that there are some leftover instances of BCH in default values for widgets.

Test Plan:
I don't know how to test this, those values are probably reinitialized in the code before the widgets are even visible, else we would have noticed the wrong "BCH" unit in the past.

I just compiled and ran bitcoin-qt to make sure everything works and looks normal.

Check that there aren't any occurences of BCH left:
grep BCH src/qt/forms/*.ui

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Subscribers: majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9921