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.

src/chat.rs

@@ -725,20 +725,11 @@ fn prepare_msg_blob(context: &Context, msg: &mut Message) -> Result<(), Error> {
     if msg.type_0 == Viewtype::Text {
         // the caller should check if the message text is empty
     } else if msgtype_has_file(msg.type_0) {
-        let blob = if let Some(f) = msg.param.get_file(Param::File, context)? {
-            match f {
-                ParamsFile::Blob(blob) => blob,
-                ParamsFile::FsPath(path) => {
-                    // path is outside the blobdir, let's copy
-                    let blob = BlobObject::create_and_copy(context, path)?;
-                    msg.param.set(Param::File, blob.as_name());
-
-                    blob
-                }
-            }
-        } else {
-            bail!("Attachment missing for message of type #{}", msg.type_0);
-        };
+        let blob = msg
+            .param
+            .get_blob(Param::File, context, !msg.is_increation())?
+            .ok_or_else(|| format_err!("Attachment missing for message of type #{}", msg.type_0))?;
+        msg.param.set(Param::File, blob.as_name());
 
         if msg.type_0 == Viewtype::File || msg.type_0 == Viewtype::Image {
             // Correct the type, take care not to correct already very special