fixes #2

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.

fixes #107

fixes #176

fixes #128

fixes #141

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

fixes #25

fixes #154

fixes #115

fixes #24

Automatically dismiss unread notifications when the window regains
focus.

fixes #111
fixes #68