gui: Fix manual coin control with multiple wallets loaded

Summary: This is a backport of Core PR18894

Test Plan:
ninja && src/qt/bitcoin-qt
Open 2 wallets, open the coin control dialog and select coins, select the second wallet and open the coin control dialog again, verify that it has been updated for the new wallet (old coins and selection cleared)

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D9098