update de_DE .po file

merged and revised version (de_DE, de_AT)
Updated dependency versions
Merge branch '2.4.x' of https://github.com/danielhjames/Airtime into 2.4.x
Merge branch '2.4.x' of https://github.com/bitdevil/Airtime into 2.4.x
CC-5246: Calendar: ON AIR show has an incorrect context menu
Merge branch '2.4.x' of dev.sourcefabric.org:airtime into 2.4.x