HexEditor: Save and load annotations to/from JSON files

This is a fairly simple JSON format: A single array, containing objects,
with the Annotation fields as key:value pairs.

When reading a file, we let invalid or missing keys fall back to the
default values. This is mostly intended to set a pattern so that if we
add new fields in the future, we won't fail to load old annotations
files. If loading the file fails though, we keep the previously loaded
set of annotations.

Userland/Applications/HexEditor/AnnotationsModel.h

@@ -65,6 +65,9 @@ public:
     void delete_annotation(Annotation const&);
     Optional<Annotation&> closest_annotation_at(size_t position);
 
+    ErrorOr<void> save_to_file(Core::File&) const;
+    ErrorOr<void> load_from_file(Core::File&);
+
 private:
     Vector<Annotation> m_annotations;
 };