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.h

@@ -36,8 +36,11 @@
 #include <LibHTML/CSS/StyleSheet.h>
 #include <LibHTML/DOM/ParentNode.h>
 
+namespace Core {
+class Timer;
+}
+
 class Palette;
-class CTimer;
 class Frame;
 class HTMLBodyElement;
 class HTMLHtmlElement;
@@ -135,7 +138,7 @@ private:
     Optional<Color> m_active_link_color;
     Optional<Color> m_visited_link_color;
 
-    RefPtr<CTimer> m_style_update_timer;
+    RefPtr<Core::Timer> m_style_update_timer;
 
     String m_source;
 };