Fix that the Custom Buttons CANNOT appear in the Navigator above.

    In the case than we need the Pager and Navigator be in both below and above the grid body, we should config "toppager" in grid options. However,when toppager is configed, Custom Buttons CANNOT appear in the Navigator above.
![navbuttonadd_0](https://f.cloud.github.com/assets/3376703/982369/5909fd34-07e8-11e3-8850-31e615e70d97.JPG)