Now you can hook activation via GAbstractView::on_activation.
The design still isn't quite right, we should eventually move the selection
away from the model somehow.
GAbstractView should be able to manage the high-level editing logic, as long
as subclasses implement content_rect(GModelIndex) so we know where to put
the editing widgets. :^)
This is pretty shaky still, but the basic idea is that you subclass GModel
and return true for editable indices. The table view also needs to have its
editable flag set.
