Improved hiding of dropdown: Instead of using negative left/right positi...

...on the dropdown visibility is now controlled by display attribute.

I want to use gwtchosen in our GWT application but found an issue. The dropdown is hidden by setting left/right property to a high negative value. This does not work correctly in my case. On first page view there are inner scrollbars because the hidden dropdowns take some place at the end of the page:
![gwtchosen_beforefirstdropdown_innerscrollbar](https://f.cloud.github.com/assets/1055217/2524106/25b8aa88-b4dc-11e3-9abf-f88e728aa9e0.png)


When I open the first dropdown the inner scrollbars disappear and the outer (default page scrollbar) appears:
![gwtchosen_afterfirstdropdown_outerscrollbar](https://f.cloud.github.com/assets/1055217/2524114/3d4a6d9e-b4dc-11e3-86de-3a8652b34faa.png)

I replaced the hiding by left/right property by using the display property.