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.

src/validationinterface.cpp

@@ -16,14 +16,16 @@
 #include <unordered_map>
 #include <utility>
 
-//! The MainSignalsInstance manages a list of shared_ptr<CValidationInterface>
-//! callbacks.
-//!
-//! A std::unordered_map is used to track what callbacks are currently
-//! registered, and a std::list is to used to store the callbacks that are
-//! currently registered as well as any callbacks that are just unregistered
-//! and about to be deleted when they are done executing.
-struct MainSignalsInstance {
+/**
+ * MainSignalsInstance manages a list of shared_ptr<CValidationInterface> callbacks.
+ *
+ * A std::unordered_map is used to track what callbacks are currently
+ * registered, and a std::list is used to store the callbacks that are
+ * currently registered as well as any callbacks that are just unregistered
+ * and about to be deleted when they are done executing.
+ */
+class MainSignalsInstance
+{
 private:
     Mutex m_mutex;
     //! List entries consist of a callback pointer and reference count. The

src/validationinterface.h

@@ -177,7 +177,7 @@ protected:
     friend class ValidationInterfaceTest;
 };
 
-struct MainSignalsInstance;
+class MainSignalsInstance;
 class CMainSignals {
 private:
     std::unique_ptr<MainSignalsInstance> m_internals;