Update qt/walletview.cpp with Qt5 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-0945de3e3345c5e5c9b39feef7843574

Test Plan:

make check
./bitcoin-qt
Settings -> Encrypt Wallet

Verify that wallet encryption works (if wallet is already encrypted, then skip this step)

Overview

Verify that clicking a transaction in Overview leads to the Transactions menu with that transaction pre-selected.

Send

Verify that after sending coins the Transaction menu is switched to with the most recent transaction selected.

Transaction

Verify that the Export button opens a pop up for exporting transaction history

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

Reviewed By: jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D4346