Added CurrentEventRow to TreeView.

This allows e.g. Toggled event handlers to know which row was changed.
It is also implemented for the Text cell renderer text changed events.
The Image and custom cell renderers were also changed to receive the TreeViewBackend, but do nothing with it at this point.
Other tree events (such as selected row changed) do not set the CurrentEventRow property.
This property is only implemented for the Gtk backend.