[core#19104] gui, refactor: Register Qt meta types in application constructor

Summary:
Backport note: Amount is not a typedef, so I moved it above the Register typedefs comment, and I removed the IMPORTANT TODO comment which has already been address (line 65: Q_DECLARE_METATYPE(Amount))

This is a backport of core#19104

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Subscribers: majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9737