Expand timeline graph and add zoom levels

The timeline graph should make it possible to better understand the temporal structure of the timeline to grasp what is located where on the timeline (visualise annotations). It should also help to navigate through the timeline, so if you for example click on an annotation-dot (or similar) in the graph, you will jump to the corresponding moment in the timeline.

Basically a vertical swimlane for the whole timeline, visualising all annotations on the timeline (which are visible in the Timeline Viewer) and indicating where annoations overlap with a video on the timeline and thus will be displayed in the Post-Annotator.

The graph should also be the basis for zooming in and out on the timeline (loading only parts of the timeline). We should keep in mind that people won't start with a very long timeline with many annotations but mostly only with 2-3 videos added over a period of 2 days. The graph and zoom-level should be thought of as growing with a project (and quite some timelines won't grow a lot but will only contain a few videos, maybe also only for one day).