It should be easy to work with a Track that starts at non-zero time.

Currently, working with a Track that has a non-zero time involves using either a Gap, or a source_range with a negative start_time, or both. This is kind of awkward. This is a common scenario and should be simpler and more intuitive.

Start by reviewing the Track time handling described in the issue, including the use of Gap objects and source_range with a negative start_time. The issue provides no file or test entry point; done means working with a Track that starts at non-zero time through a simpler, more intuitive API.

Written by the indexing model from the issue text.