Add start and optional end time for annotation tracks

[Earthwings]

• interactive markers should only appear after their start time (timestamp of first instance, usually equal to the timestamp when "New Annotation" was clicked)
• there should be an option to end an annotation track (usually when the object leaves the scene), at which point the interactive marker should disappear

[Earthwings]

Implemented in ba964fe. Annotation tracks now end automatically one second after their last commit. Similarly they start one second before their first commit, such that an extension in both dimensions (earlier start, later end) is still possible in subsequent annotation runs.