Now:
  - The current edit cell and editable widget in use can be fetched
    with properties and accessors
  - gtk_cell_area_activate_cell() handles bookkeeping of the currently
    edited cell, starting the editing of a cell, activating a cell etc
  - Exported signals are available on GtkCellArea: "editing-started",
    "editing-canceled", "editing-done", "remove-editable".
  - Upon receiving GDK_KEY_Escape current editing gets canceled.