fixes #175

fixes #157

If the user switched to another timeline before the current timeline
filled up with messages, nheko would keep fetching history.

Now it will check periodically if the timeline became visible so
it can decide whether or not to stop fetching history.

- Fix a memory leak where a layout was created without being used.
- Fix uninitialized value.

Try sending a message when all the sent messages have been received
through /sync.

fixes #115