Update german translation
Merge pull request #366 from kripton/german

Update german translation
UI: better solution for buttons background.

The Cleanlooks style doesn't exist on Qt5 anyway
New fixtures: Stairville LED Flood Panel 7x3W, Clay Paky Alpha Wash 700 (thanks to Robert Scheffler)
Release version 4.8.1.

Updated Show Manager documentation
Fixed 4.8.1 changelog date