Applications: Fix visibility of Object-derivative constructors

Derivatives of Core::Object should be constructed through
ClassName::construct(), to avoid handling ref-counted objects with
refcount zero. Fixing the visibility means that misuses like this are
more difficult.

Userland/Applications/SoundPlayer/VisualizationWidget.h

@@ -17,5 +17,6 @@ public:
     virtual void set_samplerate(int) { }
 
 protected:
+    VisualizationWidget() = default;
     virtual ~VisualizationWidget() = default;
 };