Fix synchronization between favorites view and folder tree in kmail.

1) ensure one slot doesn't call the other; the old blockSignals was meant
to do that, but that was wrong of course (the action manager doesn't
emit signals, it's connected to them)

2) set the current index in the folder tree, not just the selection,
so that keyboard navigation is more predictable, but more importantly
so that kmail is informed of the new current folder, and updates actions
appropriately.