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/src/deltachat/message.py

@@ -174,7 +174,7 @@ class Message(object):
     @property
     def _msgstate(self):
         if self.id == 0:
-            dc_msg = self.message._dc_msg
+            dc_msg = self._dc_msg
         else:
             # load message from db to get a fresh/current state
             dc_msg = ffi.gc(