onEditingStarted  
  This signal gets emitted when a cell starts to be edited. The intended use of this signal is to do special setup on @editable, e.g. adding a GtkEntryCompletion or setting up additional columns in a GtkComboBox.
See gtk_cell_editable_start_editing() for information on the lifecycle of the @editable and a way to do setup that doesn’t depend on the @renderer.
Note that GTK doesn't guarantee that cell renderers will continue to use the same kind of widget for editing in future releases, therefore you should check the type of @editable before doing any specific setup, as in the following example: |[ static void text_editing_started (GtkCellRenderer *cell, GtkCellEditable *editable, const char *path, gpointer data) { if (GTK_IS_ENTRY (editable)) { GtkEntry *entry = GTK_ENTRY (editable);
  // ... create a GtkEntryCompletion
  gtk_entry_set_completion (entry, completion);
}} ]|
Parameters
a combination of ConnectFlags
the Callback to connect. Params: editable the GtkCellEditable; path the path identifying the edited cell