Remove the "GIMP" from all "Since: GIMP 2.x" API doc comments

because it confuses gtk-doc and breaks some links. Also change the
"Index of new symbols in GIMP 2.x" sections to be what seems to be the
modern standard (looked at the GLib and GTK+ docs), and update some
other stuff.

libgimpwidgets/gimpdialog.c

@@ -114,7 +114,7 @@ gimp_dialog_class_init (GimpDialogClass *klass)
   /**
    * GimpDialog:help-func:
    *
-   * Since: GIMP 2.2
+   * Since: 2.2
    **/
   g_object_class_install_property (object_class, PROP_HELP_FUNC,
                                    g_param_spec_pointer ("help-func", NULL, NULL,
@@ -124,7 +124,7 @@ gimp_dialog_class_init (GimpDialogClass *klass)
   /**
    * GimpDialog:help-id:
    *
-   * Since: GIMP 2.2
+   * Since: 2.2
    **/
   g_object_class_install_property (object_class, PROP_HELP_ID,
                                    g_param_spec_string ("help-id", NULL, NULL,
@@ -135,7 +135,7 @@ gimp_dialog_class_init (GimpDialogClass *klass)
   /**
    * GimpDialog:parent:
    *
-   * Since: GIMP 2.8
+   * Since: 2.8
    **/
   g_object_class_install_property (object_class, PROP_PARENT,
                                    g_param_spec_object ("parent", NULL, NULL,
@@ -678,7 +678,7 @@ gimp_dialog_run (GimpDialog *dialog)
  *
  * This function is for internal use only.
  *
- * Since: GIMP 2.2
+ * Since: 2.2
  **/
 void
 gimp_dialogs_show_help_button (gboolean  show)