URLHandler: Fix opening local files
This merge request brings a fix to:
- The local files opening from the Files application that is fixed by implementing a UIDocument subclass along with the load method.
Since the UIDocument is an abstract class, we cannot directly create an UIDocument object.
We must implement our own Document subclass inheriting from UIDocument, implementing at least the load(fromContents:) method because we only do open and read files.
Success state checks are also added in order to exit the open and load methods with proper error messages.
This is a revert from 60ae1096
Closes #1530 (closed)