Using QStandardPaths::DataLocation results in broken Unicode support

A user reported a problem when creating files with our application under FreeBSD 11, where Unicode support in filenames was suddenly broken.

See: https://github.com/qTox/qTox/issues/4012

Turns out we used a deprecated enum value, which was fixed by conditionally replacing QStandardPaths::DataLocation with QStandardPaths::AppDataLocation here: https://github.com/qTox/qTox/pull/4019

However, I think using a deprecated enum value should not result in silently broken Unicode support. This should either be made more robust or the compiler should throw an error/warning.