refactor: make MainSignalsInstance() a class
Context:
MainSignalsInstance was originally a struct collection of boost::signals methods moved to validationinterface.cpp, in order to no longer need to include boost/signals in validationinterface.h.
MainSignalsInstance then evolved to remove boost/signals2 and became class-like.
C.8: Use class rather than struct if any member is non-public
C.2: Use class if the class has an invariant; use struct if the data members can vary independently
A class also has the advantage of default private access, as opposed to public for a struct.
refactor: remove unused forward declarations in validationinterface.h
Rename MainSignalsInstance() class to MainSignalsImpl()
This is a backport of core#25067