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/HTMLBlinkElement.cpp

@@ -32,7 +32,7 @@
 
 HTMLBlinkElement::HTMLBlinkElement(Document& document, const String& tag_name)
     : HTMLElement(document, tag_name)
-    , m_timer(CTimer::construct())
+    , m_timer(Core::Timer::construct())
 {
     m_timer->set_interval(500);
     m_timer->on_timeout = [this] { blink(); };