Insert new annotations as point-in-time annotations

[jduehring]

This PR should fix https://github.com/opencast/annotation-tool/issues/631.

Instead of inserting new annotations as a time based annotation with a fixed duration, they now get added as a point annotation esuring that they have a sesible width in the timeline as well as an anchor. If needed the user can of course still simply convert the annotation to a time-based one.