This fixes tons of warnings with clang from a recent xcode version on
macOS (and possibly other systems) complaining that an instantiation
of _singleton is required but no definition is available.
Registering the OSDMessageQueue instance as an event source is now done
right after the event manager is initialised. This ensures that it is created in a
sensible place and not for example in another thread). Also registering the
event source is moved to a separate function instead of being in the constructor
to remove any issue in case some code tries to display a OSD Message very early
on (the instance would be created then, but it would be registered as an event
source later).
