Fix compilation issues on Windows
Summary:
The linkage doesn't work currently on Windows because lib/test and backend/python libraries are using the same export flag than the lib/.
I have replaced the cantor_export.h flat file by calls to generate_export_header (via CMake) and put differentiated CANTOR_EXPORT/CANTOR_PYTHONBACKEND_EXPORT precompiler directives depending on the library being built.
Reviewers: Cantor, pino, asemke, sirgienko
Reviewed By: Cantor, pino, asemke, sirgienko
Subscribers: asemke, pino, sirgienko, kde-edu
Tags: KDE Edu
Differential Revision: https://phabricator.kde.org/D16039