Allow checking initialization without side-effects

Allow checking initialization without side-effects

The _clutter_context_get_default() function will automatically
create the main Clutter context; if we just want to check whether
Clutter has been initialized this will complicate matters, by
requiring a call to g_type_init() inside the client code.

Instead, we should simply provide an internal API that checks
whether the main Clutter context exists and if it has been
initialized, without any side effect.