[backport#15153] gui: Add openWallet and getWalletsAvailableToOpen to WalletController

Summary:
https://github.com/bitcoin/bitcoin/pull/15153/commits/32a8c6abfe32d5b89bf4505ca8397bdb44867614

This is a partial backport of Core PR15153

Test Plan:

ninja check

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6112