Flattened hierarchy of mainentry

This improves performances and allow us to get rid of the ```NestedWeights``` warning.
The main list now scrolls slightly faster.
Well...

![large_bug](https://cloud.githubusercontent.com/assets/3890441/4098927/1c03191c-3040-11e4-82a3-42402ba6b0f1.png)

```activity_horizontal_margin_list``` was already a custom implementation (compared with ```activity_horizontal_margin```). Why not tune it up a bit if it fixes a graphical glitch?