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/LibGUI/GFilePicker.cpp

@@ -72,7 +72,7 @@ Optional<String> GFilePicker::get_save_filepath(const String& title, const Strin
     return {};
 }
 
-GFilePicker::GFilePicker(Mode mode, const StringView& file_name, const StringView& path, CObject* parent)
+GFilePicker::GFilePicker(Mode mode, const StringView& file_name, const StringView& path, Core::Object* parent)
     : GDialog(parent)
     , m_model(GFileSystemModel::create())
     , m_mode(mode)