Add a test for bug "Partially downloaded messages are received out of order" (#3688)

+ add Message.download_state property.

python/src/deltachat/_build.py

@@ -156,6 +156,7 @@ def extract_defines(flags):
                 | DC_KEY_GEN
                 | DC_IMEX
                 | DC_CONNECTIVITY
+                | DC_DOWNLOAD
             )         # End of prefix matching
             _[\w_]+   # Match the suffix, e.g. _RSA2048 in DC_KEY_GEN_RSA2048
         )             # Close the capturing group, this contains