Re-implement dropping of files on folders in the Places Panel.

This resolves a regression caused by the Places Panel rewrite. There is
a small glitch left when reordering items (dragging below the last or
above the first item only shows the drop indicator when first dragging
out of the item and then back), but I prefer not to fix this glitch
right now because this would require a more intrusive change, and I do
not want to risk regressions because is not much time left to fix them
before 4.9.0 is released.

Thanks to Peter Penz for providing some advice about this issue.

CCBUG: 302557