Add: trigger of listview drag and drop, with scrolling
Mod: drag and drop listener now on TimedElementListView

The approach of putting the listener on each item view in the
ListAdapter was not satisfying. When the ListView recycles
item views, it works well, but when it creates a new View object
whereas the drag&drop already started, these new objects are not
elligible to receive drag&drop events.
Ref: drag & drop code reorganization
Mod: no item background transparent color, better for drag&drop
Add: visual response when dragging object on original place
Add: drop of drag&drop now implemented