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?file-filters%5B%5D=.cpp#diff-69b85bc42435f7f07306f5b22e730e0d