qt: Fix class name of Ui::ModalOverlay, Use proper class for Ui::ReceiveCoinsDialog

Summary:
Use proper classes for:

Ui::ModalOverlay to remove <customwidget> entry
Ui::ReceiveCoinsDialog to be consistent with the code base

This PR does not change behavior.

Backport of Core PR17606

Test Plan: ninja && ninja check-bitcoin-qt

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7576