Panel height adjustments.

Make panel-height auto by default.
Set max-height only on panel.
Close all other panels when opening a new one.

### Examples
![download-panel](https://cloud.githubusercontent.com/assets/96776/5037065/a4e4b8e2-6b39-11e4-9460-15903e4cf398.png)
![embed-panel](https://cloud.githubusercontent.com/assets/96776/5037066/a4e71e0c-6b39-11e4-9571-c7ab7421fec5.png)
![md-panel](https://cloud.githubusercontent.com/assets/96776/5037067/a4ea531a-6b39-11e4-902c-0cd18cdcb097.png)