new toolbar icon for liunx/win fix #2518

On black toolbars, the new icon doesn't look very well.
Now the toolbar icon has again a transparent "B" for better style on toolbars.
Does not affect the mac client.

on ubuntu it then will look like...
![Bildschirmfoto 2013-04-17 um 11 06 27](https://f.cloud.github.com/assets/178464/390377/58a1b70c-a73e-11e2-9439-47dc08d0431e.png)
![Bildschirmfoto 2013-04-17 um 11 06 15](https://f.cloud.github.com/assets/178464/390378/58a51a46-a73e-11e2-9b71-813cd1c0e176.png)