Divide Timeline into sessions and allow to expand sessions to show "commenting"-annotations

This is part of breaking down this feature issue: #1135.

If annotation of all purposes are shown in the Timeline Viewer (#1365 (closed)), you need to understand that some annotations (purpose "commenting" & "highlighting") refer to a moment in a video of an annotation with purpose "linking". If they are just listed in the same way, it will be impossible to understand it.

We already have some wireframes for this and I generally think simple lines that show the relation could be good solution, but I suggest to think of solutions which are technically easy to implement. If drawing frames around annotation which are content wise connected (and overlap on the timeline) is easier, this could also be a solution.

Additionally it could be nice to have a small overlay with the video to which the annotations you are currently scrolling through refer to. A little bit like in the "index view" of the current Motion Bank System: