Add SortedPrefixModel & SortedBufferModel
Remove SortedBufferModel (IrcBufferModel now supports sorting)
Update backend
BadgeLabel: ensure that the label remains invisible when count <= 0

Override setVisible(), because QAbstractItemView likes to control the
visibility of its index widgets, and does not respect the explicit
visibility state...
TreeView: fix storing _persistent_ indexes
TreeModel: keep persistent indexes in sync