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/FileManager/PropertiesDialog.h

@@ -40,7 +40,7 @@ public:
     virtual ~PropertiesDialog() override;
 
 private:
-    PropertiesDialog(GFileSystemModel&, String, bool disable_rename, CObject* parent = nullptr);
+    PropertiesDialog(GFileSystemModel&, String, bool disable_rename, Core::Object* parent = nullptr);
 
     struct PropertyValuePair {
         String property;