Change settings panel behavior to slide instead of grow

[UnSeulT]

This fix the animation issue mentioned by @MisterDA in #109

[jech]

I'm confused. Was that not fixed by 7aefb5e?

[UnSeulT]

It was in a way :)
This solution is cleaner and more robust as you don't have to set the width for the panel in two different files : css and in js.
The js just add/remove a class to the element and all happen in the css.
And also, using css transform is a more efficient way to deal with css animation.

[spaetz]

I have tested the merge request on top of current 0.7.1 and can attest that opening and closing the settings sidebar works nicely on Firefox. Plus it is 11 lines of code less, so +1 for merging :)