Use one apple-touch-icon instead of six

This commit removes five Apple Touch Icons with different sizes for
the same icon and adds a new icon `apple-touch-icon-precomposed.png`
with a resolution of 152x152px.
Apart from that it adds documentation on how to add more icons if
you need them within your project.
Using only one Apple Touch Icon saves time and is enough in most
cases.

Closes #1367.