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/DisplayProperties/DisplayProperties.h

@@ -65,7 +65,7 @@ private:
 
 private:
     String m_wallpaper_path;
-    RefPtr<CConfigFile> m_wm_config;
+    RefPtr<Core::ConfigFile> m_wm_config;
     RefPtr<GWidget> m_root_widget;
     Vector<Size> m_resolutions;
     Vector<String> m_wallpapers;