Remove unused structure _AtkEditableTextIfaceClass

Remove unused structure _AtkEditableTextIfaceClass

* atk/atkeditabletext.c:
Remove unused structure _AtkEditableTextIfaceClass

* atk/atkobject.[c|h]
Remove signals model_changed and selection_changed

* atk/selection.[c|h]
Add signal selection_changed

* atk/atktable.[c|h]
Remove unused structure _AtkTableIfaceClass
Add signal model_changed

* atk/atktext.[c|h]
Remove unused structure _AtkTextIfaceClass
Add signal text_selection_changed

* docs/tmpl/atkobject.sgml, docs/tmpl/atkselection.sgml,
docs/tmpl/atktable.sgml, atk/tmpl/atktext.sgml:
Documentation changes