Merge #15101: gui: Add WalletController

Summary:
0dd9bdefa gui: Refactor to use WalletController (João Barbosa)
8fa271f08 gui: Add WalletController (João Barbosa)
cefb399e2 gui: Use AutoConnection for WalletModel::unload signal (João Barbosa)

Pull request description:

This PR is a subset of the work done in the context of #13100. This change consists in extracting from the application class the code that manages the wallet models.

The role of the `WalletController` instance is to coordinate wallet operations and the window.

Tree-SHA512: 6a824054376730eb7d16c643dd2003f5f60778e8ad3af707b82bc12c48438db179ca4446316b28fb17b206f4b9aba8998419aab8c5dd1f7c32467015732b5094

Backport of Core PR15101
https://github.com/bitcoin/bitcoin/pull/15101/files
Completes T697
Depends on D4619

Test Plan:

cmake -GNinja ..
ninja check

cmake -GNinja -DBUILD_BITCOIN_WALLET=OFF ..
ninja check

../configure
make check

../configure --disable-wallet
make check

Reviewers: #bitcoin_abc, deadalnix, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D4620