Field Definition: Move Default formatting into a separate window.

* glom/mode_dsign/fields/dialog_fielddefinition.[h|cc]:
* data/ui/developer/window_field_definition_edit.glade:
  Move the Default Formatting notebook tab into:
* glom/mode_design/fields/dialog_defaultformatting.[h|cc]:
* data/ui/developer/window_default_formatting.glade:

* glom/utility_widgets/adddel/adddel.[h|cc]:
  Add signal_user_requested_extra.
* glom/utility_widgets/adddel/adddel_withbuttons.[h|cc]:
  Add set_edit_button_label() and set_extra_button_label()
  and override show_all_vfunc() to not show the extra button if
  it has no label.

* glom/mode_design/fields/box_db_table_definition.[h|cc]:
  Use the new button to show the new dialog.