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-7285193040d20d3f795694215e083efc