Merge branch 'alvin/T6503-python-win'

This also merges branch 'alvin/T6521-deps-update-win' into 'master'.

This merge enables Python scripting on Windows and updates some of the
3rdparty dependencies on 'master'. You should do a complete clean
rebuild of everything on Windows with the latest mingw-w64 7.1 compiler
to enable it.

CCMAIL: kimageshop@kde.org
Maniphest Tasks: T6503, T6521