project_panel: Add directory auto-expand after 500ms hover during dragging

[0xtimsb]

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.

auto-expand.mp4

Release Notes:

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

[ConradIrwin]

Thanks for this!

The timeout might be a bit short, but let's try it out and see if it causes problem in real life.

[JosephTLyons]

Just compiled main and tried this and it's such a nice QoL improvement! Thanks Smit!