Fix non-existent "anyDragAndDrop" property usage in Panel main.qml
This removes a very common "cannot assign undefined to bool" warning.
cc @niccolove 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 source code. I assume it was meant to be root.dragAndDropping
so I changed it to that. I didn't observe a functional difference either way, but probably I wasn't looking in the right place.