Begins editing on a this.
The `GtkCellRenderer` for the cell creates and returns a `GtkCellEditable` from start_editing, configured for the `GtkCellRenderer` type.
start_editing can then set up this suitably for editing a cell,
e.g. making the Esc key emit `GtkCellEditable:GtkCellEditable:editing-done`.
Note that the this is created on-demand for the current edit; its lifetime is temporary and does not persist across other edits and/or cells.
| this |
A `GtkCellEditable` |
| event |
The `GdkEvent` that began the editing process, or null if editing was initiated programmatically |