[backport#15450 4/5] Expose wallet creation to the GUI via WalletController

Summary:
Co-authored-by: João Barbosa <joao.paulo.barbosa@gmail.com>

https://github.com/bitcoin/bitcoin/pull/15450/commits/9b41cbb28f603f4f71f5854d6ae2527932bba3cb

Depends on D7104

Partial backport of Core PR15450

Test Plan:

ninja check

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7105