Only enable slider behaviour if there are at least two panes.

@4teamwork/coders @maethu
Could someone take a look please? :eyes: 

If there is just one SliderPane there is no need to enable the slider behaviour.
![slider](https://f.cloud.github.com/assets/157533/1866777/0b66feae-783d-11e3-8302-e9b7311c1b9a.png)