EngineBase: Use fully qualified namespace in StateChanged

Makes sure the metatype matches with Qt 5:
QObject::connect: Cannot queue arguments of type 'State'
(Make sure 'State' is registered using qRegisterMetaType().)

Fixes #1446

src/engine/enginebase.h

@@ -184,7 +184,7 @@ class EngineBase : public QObject {
 
   // Signals that the engine's state has changed (a stream was stopped for example).
   // Always use the state from event, because it's not guaranteed that immediate subsequent call to state() won't return a stale value.
-  void StateChanged(const State state);
+  void StateChanged(const EngineBase::State state);
 
   void VolumeChanged(const uint volume);