Another approach to fix bug 291068, be more permissive

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.


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