Panel: Fix non-existent "anyDragAndDrop" property usage

This removes a very common "cannot assign undefined to bool" warning.

The use of this `anyDragAndDrop` property was introduced in
4726b9dd, but as far as I can tell this
is the only reference to this property anywhere in KDE sourcecode. It
was meant to be `root.dragAndDropping` so I changed it to that.