LibCore: Put all classes in the Core namespace and remove the leading C

I've been wanting to do this for a long time. It's time we start being
consistent about how this stuff works.

The new convention is:

- "LibFoo" is a userspace library that provides the "Foo" namespace.

That's it :^) This was pretty tedious to convert and I didn't even
start on LibGUI yet. But it's coming up next.

Libraries/LibGUI/GSlider.cpp

@@ -162,7 +162,7 @@ void GSlider::mouseup_event(GMouseEvent& event)
     return GWidget::mouseup_event(event);
 }
 
-void GSlider::leave_event(CEvent& event)
+void GSlider::leave_event(Core::Event& event)
 {
     if (!is_enabled())
         return;