treeview: Replace cursor handling by keeping the node

treeview: Replace cursor handling by keeping the node

Previously the code used a GtkTreeRowReference, which was (a) less
performant and more importantly (b) hiding errors.
The errors being hidden were the referenced row becoming invalid or
collapsed, because such rows would not be valid cursor rows and it would
be necesary to select a new row and emit a "cursor-changed" signal.

So if a crash bisects down to this commit, it is very likely that the
cursor has not correctly been updated and the cursor row is invalid.