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/GAbstractTableView.cpp

@@ -517,7 +517,7 @@ void GAbstractTableView::context_menu_event(GContextMenuEvent& event)
         on_context_menu_request(index, event);
 }
 
-void GAbstractTableView::leave_event(CEvent&)
+void GAbstractTableView::leave_event(Core::Event&)
 {
     window()->set_override_cursor(GStandardCursor::None);
     set_hovered_header_index(-1);