treeview: Ensure selection's changed signal

treeview: Ensure selection's changed signal

There was a corner case where the changed signal was not emitted.
If rows were built like this:
  1    (not selected)
  + 2  (selected)
  + 3  (not selected)
And row 1 was removed, no signal would be emitted.

I like it when writing tests actually finds bugs that have been around
since 2003 - introduced by 4a03ea2334bde7d2e15d0933c5a78f5a2338a854
actually. :)