C++ events are now enum instead of const static int

[GiuseppeCesarano]

This pr is not intended to change API compatibility, in fact the C++ example serve_a_folder can be compiled without changes, but a larger discussion should be held about the status of enums for example:

• Most enums are PascalCase without repetitions
• The event enum is uppercase snake_case with repetitions
• The event enum is of type enum(int), but is stored in a size_t
• Language bindings sometimes redeclare enums with a different base type

[AlbertShown]

No breaking changes, but a good improvement, thank you @GiuseppeCesarano 👍

I personally likes enum because it can enable certain compile-time checks and warnings over constants plain integers, also debuggers can show the names instead of integer.

[hassandraga]

Thank you @GiuseppeCesarano.