Project Panel drag-and-drop usability improvements

[notpeter]

Description

Dragging and dropping in the project panel is pretty primitive at the moment. Would be nice to have some features that make it behave more like Finder or VSCode:

• [x] While dragging, support pressing escape to cancel
• [x] While dragging, if you hover over a folder for ~500ms, folder should auto-expand so you can drag-and-drop into nested folders.
• [ ] When initiating a drag the blue selection rectangle should jump to the item you've picked up (so you can know which file you've grabbed)
• [x] While dragging, currently whatever is under the mouse cursor (other files, etc) is highlighted with an inverted rectangle. Instead the containing folder which would be the target of the drop operation should highlight.
• [x] Holding alt or shift to copy the file should add a green (+) icon to the mouse cursor to indicate this is a copy operation.