Post-Annotator: add switch button if there are overlapping videos

In case one or more videos overlap with the video that you are looking at in the Post-Annotator a switch-button should allow you to select to which video you want to switch. This will make it easier to switch between different perspectives of the same recording session.

Suggestions in issue #850 (closed) could be implemented but we need to very careful to not overload the interface in the post-annotator. I think a button that is only visible in case there is something to switch to can be sufficient and if it's more than one other video you could select the perspective/video in a dialog.