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.

Applications/HexEditor/HexEditor.cpp

@@ -564,7 +564,7 @@ void HexEditor::paint_event(GPaintEvent& event)
     }
 }
 
-void HexEditor::leave_event(CEvent&)
+void HexEditor::leave_event(Core::Event&)
 {
     ASSERT(window());
     window()->set_override_cursor(GStandardCursor::None);