icons: updated icon-theme icons.

Current icon uses stars for symbolizing an icon. Stars are used normally
for rating.
My suggestion uses yin/yang, why ?
It's difficult to make an icon which symbolizes an icon. So I decided to
use the world-wide most used symbol (non-religious, non-political...) as
a symbol for all symbols (this is quite similar to e.g. the usage of
mona-lisa as a symbol for all images).