move buttons to the bottom of the filter form

The apply/reset button of the filter form, they float right on the window, it is too far away from the filter-fields-area. Especially when using wide-screen monitor.

 It look better if we put them below the filter-fields-area.

Preview

![selection_003](https://f.cloud.github.com/assets/330812/1680731/1e5fd8da-5d82-11e3-83d6-b5c193f7277a.png)