Change DnD functionality with nested spans implementation

[Dananji]

Description

After the re-work of the structure display and edit functionality in SME to support nested timespans, the DnD (Drag-n-Drop) functionality does not provide accurate drop-zone calculation.
In the previous implementation supporting non-overlapping timespans, the drop-zones were calculated by only considering heading type structure items.
But, with the latest changes to support nested timespans it seems the drop-zone calculation probably should be extended consider possible parent timespans as well.

The EVIADA example record could be used for testing this: https://avalon-dev.dlib.indiana.edu/media_objects/12579s29x

Done Looks Like

A timespan can be dragged and dropped inside a,

• heading item if the structure inside that heading is not disturbed by the new timespan, i.e. suggested drop zone maintains the chronological order of the timespans
• timespan if the parent timespan fully encloses the dragged timespan's range without overlapping its children

[joncameron]

Should be able to reuse a lot of the existing functionality to provide required values for this.

What tests exists for drag and drop? We should look at that as well.