[ci skip]

I had a look at sync.cpp and checked which parts of the sync response
are currently handled and which not. As I think it is unnecessary to let the 
unhandled data be transmitted without being handled I added these filters.

In the same term I increased the timeout server-side to 30s as Riot
defaults to this value as well. Especially now when a lots of presence-updates 
are not send anymore this value is more relevant.

It is now also possible to use a filter that is defined in`client/sync_filter`.
Advanced users might want to set an own filter here.

[ci skip]

fixes #132 

[ci skip]

fixes #175

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 #113
fixes #173

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

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