Update qt/rpcconsole with Qt5 connect syntax

Summary:
Updates to use Qt5 connect syntax over Qt4.
See https://wiki.qt.io/New_Signal_Slot_Syntax for details.

Partial backport of Core PR13529
https://github.com/bitcoin/bitcoin/pull/13529/commits/f78558f1e39198779bdb17e2b0e256fb99ad4b28#diff-2c15c5b52f35ea388ebab757eaab0f1c

Test Plan:

make check
./bitcoin-qt
Help -> Debug -> Console

Input some commands to make sure the console works.

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D4325