Add autosave feature. #45

During more lengthy annotations, crashes might occur (either related or unrelated to Python Video Annotator), such that a lot of work might be lost if the user does not save often, as is the case when one is focused on the current task.

It would be a great addition to have an autosave feature, which would save the current work at a fixed timed interval to a default directory. Both the time interval and save directory could be hard coded, but ideally could be implemented to be user-defined.