Drag / Drop behaviour for QListWidget

BUG: to reorder the Markers order, there is no SIGNAL for Drag/Drop in
QListWidget. Therefore the program doesn't know the order of the markers
after drag/drop. Very annoying.

Maybe a QT extension exist that has this SIGNAL behaviour for
QListWidget?

For now, we run `reorderMarkers` at any button clicked. (overload,
uggh...)