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/chat.py

@@ -109,6 +109,30 @@ class Chat(object):
 
     # ------  chat messaging API ------------------------------
 
+    def send_msg(self, msg):
+        """send a message by using a ready Message object.
+
+        :param msg: a :class:`deltachat.message.Message` instance
+           previously returned by
+           e.g. :meth:`deltachat.message.Message.new_empty` or
+           :meth:`prepare_file`.
+        :raises ValueError: if message can not be sent.
+
+        :returns: a :class:`deltachat.message.Message` instance as
+           sent out.  This is the same object as was passed in, which
+           has been modified with the new state of the core.
+        """
+        if msg.is_out_preparing():
+            assert msg.id != 0
+            # get a fresh copy of dc_msg, the core needs it
+            msg = Message.from_db(self.account, msg.id)
+        sent_id = lib.dc_send_msg(self._dc_context, self.id, msg._dc_msg)
+        if sent_id == 0:
+            raise ValueError("message could not be sent")
+        # modify message in place to avoid bad state for the caller
+        msg._dc_msg = Message.from_db(self.account, sent_id)._dc_msg
+        return msg
+
     def send_text(self, text):
         """ send a text message and return the resulting Message instance.
 
@@ -130,9 +154,12 @@ class Chat(object):
         :raises ValueError: if message can not be send/chat does not exist.
         :returns: the resulting :class:`deltachat.message.Message` instance
         """
-        msg = self.prepare_message_file(path=path, mime_type=mime_type)
-        self.send_prepared(msg)
-        return msg
+        msg = Message.new_empty(self.account, view_type="file")
+        msg.set_file(path, mime_type)
+        sent_id = lib.dc_send_msg(self._dc_context, self.id, msg._dc_msg)
+        if sent_id == 0:
+            raise ValueError("message could not be sent")
+        return Message.from_db(self.account, sent_id)
 
     def send_image(self, path):
         """ send an image message and return the resulting Message instance.
@@ -142,9 +169,12 @@ class Chat(object):
         :returns: the resulting :class:`deltachat.message.Message` instance
         """
         mime_type = mimetypes.guess_type(path)[0]
-        msg = self.prepare_message_file(path=path, mime_type=mime_type, view_type="image")
-        self.send_prepared(msg)
-        return msg
+        msg = Message.new_empty(self.account, view_type="image")
+        msg.set_file(path, mime_type)
+        sent_id = lib.dc_send_msg(self._dc_context, self.id, msg._dc_msg)
+        if sent_id == 0:
+            raise ValueError("message could not be sent")
+        return Message.from_db(self.account, sent_id)
 
     def prepare_message(self, msg):
         """ create a new prepared message.

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(