[Atlas] Update spacing for atlas panel

Hi Hugo,

To follow up your commit with the collapsible group boxes, I worked on the spacing a bit so that it more closely matches the other panels.

Also, what do you think about integrating the Atlas panel within the Composition panel, instead of having it as a separate one? I did this in a very quick manner to show you how it might look (both Composition and Atlas take up 50% vertically in this layout):

http://drive.dakotacarto.com/qgis/composer_atlas-composition.png

Are there reasons why such an integration into the Composition panel should not be considered? As a dock, it automatically offers a QSplitter, but, IMHO, having it inside the Composition panel is cleaner and give the functionality more exposure to users.

Regards,

Larry