Add switch button to Timeline Viewer to show all annotations
This is part of breaking down this feature issue: #1135
The "expand"/"collapse" button should allow you to switch between seeing all annotations (all purposes) on a Timeline and only those with purpose "linking" (current Timeline Viewer).
Annotations with purpose "commenting" and "highlighting" should probably show up below each other even if they "overlap", but we can check if it makes sense to show them next to each other if they have the same start time.