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/LibHTML/DOM/Document.cpp

@@ -47,7 +47,7 @@ Document::Document()
     : ParentNode(*this, NodeType::DOCUMENT_NODE)
     , m_style_resolver(make<StyleResolver>(*this))
 {
-    m_style_update_timer = CTimer::construct();
+    m_style_update_timer = Core::Timer::construct();
     m_style_update_timer->set_single_shot(true);
     m_style_update_timer->set_interval(0);
     m_style_update_timer->on_timeout = [this] {