Update qt/walletmodel.cpp 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-2e3836af182cfb375329c3463ffd91f8

Test Plan:

make check
./bitcoin-qt
Send -> Receive -> Transactions

Verify each sub menu loads correctly.

Help -> Debug -> Console
createwallet test

Switch to the new wallet in both the console and the main window
In the console:

getwalletinfo

In the mainwindow:

Overview -> Send -> Receive -> Transactions

Verify the main window loadseach sub menu correctly.

Reviewers: deadalnix, Fabien, jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Reviewed By: Fabien, O1 Bitcoin ABC, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D4343