gui refactor: Inline SplashScreen::ConnectWallet

Summary:
Suggested https://github.com/bitcoin/bitcoin/pull/19099#discussion_r450522201

This concludes backport of core#19099 [3/3]
https://github.com/bitcoin/bitcoin/pull/19099/commits/24bf17602c620445f76c3b407937751c8a894d37

Depends on D10142

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10143