Addresses hang issue seen in T28 by freetrader -- properly register Config type with Qt so it may be used in signals/slots

Summary: T28 issue was that passing Config object around confused Qt's type system so initialize() slot was never invoked. This fixes it to work.

Test Plan: build with Qt and run src/qt/bitcoin-qt. If it gets past splash screen without hanging, it works.

Reviewers: #bitcoin_abc, deadalnix, freetrader, sickpig, kyuupichan, awemany, O1 Bitcoin ABC

Reviewed By: #bitcoin_abc, freetrader, O1 Bitcoin ABC

Maniphest Tasks: T28

Differential Revision: https://reviews.bitcoinabc.org/D292