refactor: make MainSignalsInstance() a class

and use Doxygen documentation for it, per our developer notes.

Context:

MainSignalsInstance was created in 3a19fed9db5 and originally was 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 in d6815a23131 to remove boost/signals2 and became class-like.

[C.8: Use class rather than struct if any member is
non-public](https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#Rc-class)

[C.2: Use class if the class has an invariant; use struct if the data members can vary
independently](https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#c2-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.

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