Bart originally solved the bug by enabling track dropping only
precisely on root collection rows. This is IMO too much restrictive
as it prevents you to drag tracks between 2 large expanded collections
without excessive scrolling. (something I would miss) Additionally,
there was no visual indication that the drop will not be performed.

This is my try to rework it in a way that:
 * keeps drops onto artists/albums (whatever you first level
   entity in collection browser is) allowed. There is a drop indicator
   that clearly shows that the drop will go _between_ the entities, not
   to them.
 * disables drops to read-only collections
 * disabled drops are indicated visually using the not-allowed mouse
   cursor (the tricky part, but commented well in code)

Some more code comments not present in review request are added.

BUG: 291068
REVIEW: 103856
DIGEST: More natural drag'n'drop behaviour in collection browser