bug 1266496 - fire some selection events for proxied accessibles r=davidb

we map both EVENT_SELECTION_CHANGED and EVENT_TEXT_SELECTION_CHANGED to
"selection_changed" similar to the non proxy case.

accessible/atk/AccessibleWrap.cpp

@@ -1495,6 +1495,10 @@ a11y::ProxyEvent(ProxyAccessible* aTarget, uint32_t aEventType)
   case nsIAccessibleEvent::EVENT_VALUE_CHANGE:
     g_object_notify((GObject*)wrapper, "accessible-value");
     break;
+  case nsIAccessibleEvent::EVENT_TEXT_SELECTION_CHANGED:
+  case nsIAccessibleEvent::EVENT_SELECTION_WITHIN:
+    g_signal_emit_by_name(wrapper, "selection_changed");
+    break;
   }
 }