Add a GetStandardProperties trait.

src/parser/paragraph.rs

@@ -74,7 +74,7 @@ mod tests {
     use crate::context::List;
     use crate::parser::element_parser::element;
     use crate::parser::org_source::OrgSource;
-    use crate::types::Source;
+    use crate::types::GetStandardProperties;
 
     #[test]
     fn two_paragraphs() {
@@ -87,7 +87,13 @@ mod tests {
         let (remaining, second_paragraph) =
             paragraph_matcher(remaining).expect("Parse second paragraph.");
         assert_eq!(Into::<&str>::into(remaining), "");
-        assert_eq!(first_paragraph.get_source(), "foo bar baz\n\n");
-        assert_eq!(second_paragraph.get_source(), "lorem ipsum");
+        assert_eq!(
+            first_paragraph.get_standard_properties().get_source(),
+            "foo bar baz\n\n"
+        );
+        assert_eq!(
+            second_paragraph.get_standard_properties().get_source(),
+            "lorem ipsum"
+        );
     }
 }