Re-work some in-creation file handling

This effectively reverts
https://github.com/deltachat/deltachat-core-rust/pull/964 for chat.rs,
which in that PR was thought to fix something.  So maybe something is
still broken?  But after improving tests the previous code seems to be
correct.

- Update Python bindings to not always use dc_prepare_msg path when
  sending messages with attachements.  When using dc_prepare_msg the
  blobs need to be created in the blobdir since they will not get
  copied and many tests where not doing this.

- Add a test that ensures that calling dc_prepare_msg with a
  file **not** in the blobdir fails.

- Add a test that ensures that calling dc_send_msg directly with a
  file **not** in the blobdir copies the file to the blobdir.  This
  test cheats a little by knowing what the filename in the blobdir
  will be which is implementation-dependent and thus a bit brittle.
  But for now it proves correct behaviour so let's go with this.

- Improve the test_forward_increation test to ensure that the
  in-creation file only has it's final state before calling
  dc_send_msg.  This checks the correct file data is sent out and not
  the preparing data, this fails with the chat.rs changes in
  #964 (reverted here to make this work again).  Also fix the test to
  actually create the in-creation file in the blobdir.

- Fix test_send_file_twice_unicode_filename_mangling to not use
  in-creation.  It was not creating it's files in the blobdir and that
  is an error when using in-creation and it didn't seem it was trying
  to test something about the in-creation logic (which is tested in
  test_increation.py already).

- Fix Message._msgtate code which presumably was not used before?

- Rename `BlobObject::create_from_path` to
  `BlobObject::new_from_path`.  All the `BlobObject::create*` calls
  now always create new files which is much more consistent.  APIs
  should do what is obious.

python/tests/test_account.py

@@ -456,10 +456,7 @@ class TestOnlineAccount:
             msg1 = Message.new_empty(ac1, "file")
             msg1.set_text("withfile")
             msg1.set_file(p)
-            message = chat.prepare_message(msg1)
-            assert message.is_out_preparing()
-            assert message.text == "withfile"
-            chat.send_prepared(message)
+            chat.send_msg(msg1)
 
             lp.sec("ac2: receive message")
             ev = ac2._evlogger.get_matching("DC_EVENT_INCOMING_MSG|DC_EVENT_MSGS_CHANGED")