Grid accessibility: incorrect focus can be read

Problem: this affects NVDA and Narrator, but not Jaws. If a user moves to the Delete button and presses it to delete a label, a label is incorrectly read as the new focus - the focus remains on the Delete button.
GridAx::SetCurrentCell() can be called when the Grid is not the focus, and send a focus event.

Fix: In GridAx::SetCurrentCell() only send an focus event if the Grid is the focus.

src/widgets/Grid.cpp

@@ -739,10 +739,12 @@ void GridAx::SetCurrentCell(int row, int col)
                mLastId);
    }
 
-   NotifyEvent(wxACC_EVENT_OBJECT_FOCUS,
-               mGrid->GetGridWindow(),
-               wxOBJID_CLIENT,
-               id);
+   if (mGrid == wxWindow::FindFocus()) {
+      NotifyEvent(wxACC_EVENT_OBJECT_FOCUS,
+                  mGrid->GetGridWindow(),
+                  wxOBJID_CLIENT,
+                  id);
+   }
 
    NotifyEvent(wxACC_EVENT_OBJECT_SELECTION,
                mGrid->GetGridWindow(),