project_panel: Add directory auto-expand after 500ms hover during dragging (#23080)

This PR resolves one part of issue #14496 

In project panel, when dragging, if you hover over a directory for
~500ms, it now auto-expands so you can drag and drop into nested
directories.

Task cleanup is handled in these cases:  
- Dragged onto a different entry.  
- Dragged anywhere else, and the 500ms timer runs out (for example, out
of the project panel).
- Dropped onto any entry.  

I don’t see any edge cases where task isn’t cleaned up after 500ms.


https://github.com/user-attachments/assets/19da0da1-f9e2-42df-8ee4-fab6dc9a185a

Release Notes:

- Added auto-expand for directories on hover for a while during
dragging.