[backport#14177] qt: Set C locale for amountWidget

Summary:
b0510d78aedde864756199fe71ca98f8e95dd44f Set C locale for amountWidget (Hennadii Stepanov)

Pull request description:

Fix #13873

Backport of Core PR14177

Test Plan:

ninja all
./qt/bitcoin-qt -testnet

move to transactions tab, try set an amount filter using comma instead of point and make sure I fail to do so.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D8545