Add support for deletion of parent timespans

[Dananji]

Description

After the re-work of the structure display and edit functionality in SME to support nested timespans, the deletion of timespans needs to be re-worked.

In the previous implementation, timespans were considered leaf nodes. Therefore deletion of a timespan only removes itself from the structure and Peaks instance.
But, with the latest changes timespan deletion needs to recursively go through its child timespans and delete its children similar to a deletion of a heading item.
The EVIADA example record could be used for testing this: https://avalon-dev.dlib.indiana.edu/media_objects/12579s29x

Done Looks Like

• When a timespan with children is deleted its children are removed from both structure and Peaks (in the waveform)
• When a timespan without children is deleted only that particular timespan is removed from both structure and Peaks