Update de.ts

Corrections (some of them according to the comments from 'nachfuellbar'). The vanished translation too, may be reactivating them.
prevents division by 0 in updater
GroupList refactoring
auto-update audio devices list on tab show
Group class refactoring
using ToxID in groupchats
GroupWidget: menu reordering
groupchats: friend's aliases (updates on rejoin)
commented currently unused functions in Group class
rescan audio devices button
minor fixes in ChatAction's creation/processing
Update de.ts "calling" will be "ruft an"
added opencv imgproc to dependencies


```
$ ./qtox 
./qtox: error while loading shared libraries: libopencv_imgproc.so.2.4: cannot open shared object file: No such file or directory
```
-P value name also translated
Merge branch 'activate-existing-instance' into custom
Removed explanations from system tray menu
Merge branch 'pr839'
Merge branch 'pr845'
Merge branch 'pr846'
Merge branch 'pr848'