Merge #14784: qt: Use WalletModel* instead of the wallet name as map key

Summary:
91b0c5b09616bb8c5ef8107efb432ce4c7c9a383 qt: Use WalletModel* instead of wallet name in console window (João Barbosa)
b2ce86c3ad79db49ddf06207230512300ec11156 qt: Use WalletModel* instead of wallet name in main window (João Barbosa)
d2a1adffebee86763fafc6f3ed2722f7038654c7 qt: Factor out WalletModel::getDisplayName() (João Barbosa)

Pull request description:

This a small refactor that doesn't change behavior. This is also necessary if in the future we allow renaming wallets.

Tree-SHA512: 1820d0ff28e84b1d862097f1f55b52f94520fa50c9b1939d235a448a48159748c3bbf99b19e4cb1ff4f91efc008c0971b4c25a91f645f9d43792c8aeaa93cf9e

Backport of Core PR14784
https://github.com/bitcoin/bitcoin/pull/14784/files

Completes T698

Test Plan: ninja check

Reviewers: #bitcoin_abc, deadalnix, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D4567