changed release version to 1.6.4

changed version to 1.6.3

updated german translations

Merge branch 'dev-qt5.1x' of https://github.com/OpenBoard-org/OpenBoard…

… into dev-qt5.1x

updated release_scripts with Qt 5.15.2

updated version number to 1.6.1a-1102

Merge branch 'dev-qt5.1x' of https://github.com/OpenBoard-org/OpenBoard…

… into dev-qt5.1x

updated version to 1.6.0a2

added quick reminder on types

restrict workaround to 10.13+ versions