qt: Use new Qt5 connect syntax
Upgrades the gui code from using Qt4 SIGNAL/SLOT syntax to Qt5's new
syntax.
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
Depends on D4304