add a data_dragend callback that can return a selection extent

I would like to add an optional callback that can provide the selection extent back to the user. Useful to determine the selection domain.

No files or tests are named. Start by locating the existing data_drag callbacks and selection extent handling in the C3 codebase, then define how the optional callback receives the extent and add coverage showing that data_dragend exposes the selected domain.

Written by the indexing model from the issue text.