qt: Add WalletFrame::sizeHint

Summary:
This change makes the layout of a WalletFrame widget correct even without loaded wallets.
Previously, the window was very small, and as a result the progress dialog overlay was invisible.

This is a backport of core-gui#116

Test Plan: src/qt/bitcoin-qt -regtest -resetguisettings -nowallet

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10680