on &action=edit.
$EditPage : the EditPage object
+'AlternateSkinPreferences': before creating the content of the 'skin'
+tab in preferences
+$prefsform : the PreferencesForm object
+
'APIEditBeforeSave': before saving a page with api.php?action=edit,
after processing request parameters. Return false to let the request
fail, returning an error message or an <edit result="Failure"> tag