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.

Signed-off-by: Jonas Schnelli <jonas.schnelli@include7.ch>
Merge pull request #2537 from jonasschnelli/toolbaricon_linux

new toolbar icon for liunx/win fix #2518