Improve contrast of app icons

We heard feedback that it was too difficult to disambiguate icons in the new menu. To be able to more easily tell icons apart from each other, we should flip the colors:

• The foreground color should be derived from primary
• The background gradient from secondary
• The shadow and top shine should be removed
• And we can also slightly increase the size of icons and the circles they're in

<img width="1148" height="984" alt="Image" src=" />

cc @pringelmann @jancborchardt