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/Piano/MainWidget.cpp

@@ -70,7 +70,7 @@ MainWidget::~MainWidget()
 // FIXME: There are some unnecessary calls to update() throughout this program,
 // which are an easy target for optimization.
 
-void MainWidget::custom_event(CCustomEvent&)
+void MainWidget::custom_event(Core::CustomEvent&)
 {
     m_wave_widget->update();