UI: Small alignment changes on the homescreen

- changed the scroll bar to the wider version
- removed the padding around the whole screen
- added a bit of padding to the menu-buttons

The idea is to make it consistent with kano-profile
and the new redesign of kano-apps.

Related to: https://github.com/KanoComputing/peldins/issues/1258

Here's how it looks now:

![refactored-home-screen](https://cloud.githubusercontent.com/assets/169328/4058318/7acf46c8-2dd4-11e4-9e9f-47f268112cb2.png)

cc @carolineclark @TommySaehl @alex5imon