Simplify drop event logic in `sub_plot_widget.py`

[miker2]

The dropEvent method has become quite long and handles several distinct cases (reordering within the same subplot, moving a plot item from another subplot, and adding a new item from VarListWidget). This complexity can make the method harder to understand, debug, and maintain.

Would it be possible to refactor this method by extracting the logic for each main scenario into its own private helper method? For example:

• _handle_custom_plot_item_drop(self, event, mime_data, plot_name, actual_source_widget, idx_for_insertion)
  • This could then further delegate to _handle_reorder_within_subplot(...) and _handle_move_between_subplots(...).
• _handle_data_item_drop(self, event, mime_data)

This could improve readability and separation of concerns.

Originally posted by @gemini-code-assist[bot] in #2 (comment)