The tree model nodes consist of GNode structs:

struct GNode {
  gpointer data;
  GNode	  *next;
  GNode	  *prev;
  GNode	  *parent;
  GNode	  *children;
};

where children are a linked list. To append a value, the list has to be
walked to the end and with nodes with many children (which is our case)
this becomes very expensive.

We sort the tree afterwards anyway so it doesn't matter where we insert the
value.