EventMetaType, QUO_EVENT, QUO_BASE_EVENT

The new metatype framework replaces
EventFactory/DEFINE_EVENT_TYPEID/REGISTER_EVENT_TYPE; it is faster,
more functional and extensible. Of note:
- EventMetaType mostly reproduces the logic of EventFactory but supports
  custom base event types not just for loading (that part EventFactory
  also supported) but also for matching - previously you had to have
  Event::is*Event() for base type matching. Now Quotient::is() can
  match against both base and leaf types.
- Instead of DEFINE_EVENT_TYPEID and REGISTER_EVENT_TYPE there's now
  a single macro, QUO_EVENT, intended for use in the way similar to
  Q_OBJECT. Actually, the entire framework borrows heavily from
  QMetaObject and Q_OBJECT. Making event types full-fledged QObjects
  is still not considered because half of QObject functions would not
  be applicable (e.g. signals/slots) while another half (in particular,
  using Matrix type ids to select event types) would still have to be
  done on top of QObject. And QML can just access events as
  const QJsonObjects which is arguably more lightweight as well.
- QUO_BASE_EVENT is a new macro replacing EventFactory object
  definitions. This was necessary for the same reason why Q_OBJECT is
  a macro: aside from a static object definition, this macro
  introduces a virtual function override to resolve the metatype at
  runtime. This very mechanism is used to make event type
  matching/casting as quick as possible
- QUO_BASE_EVENT and QUO_EVENT use the C++20 __VA_OPT__ feature that
  is only available with the new MSVC preprocessor (see
  https://docs.microsoft.com/en-us/cpp/preprocessor/preprocessor-experimental-overview);
  the respective switch was added to CMakeLists.txt.

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