* src/CMakeLists.txt (qgpgme_PRIVATE_HEADERS): Add new private header
file.
* src/exportjob.cpp, src/exportjob.h (class ExportJob): Add private
class as argument to protected constructor. Add member functions
setExportFilter and exportFilter.
* src/exportjob_p.h: New.
* src/qgpgmeexportjob.cpp (QGpgMEExportJobPrivate): New.
(export_qba): Add arg exportFilter and adjust the callers. Set
export-filter context flag.
* src/qgpgmeexportjob.h (QGpgMEExportJob): Specify private class as
template argument of ThreadedJobMixin.
* tests/run-exportjob.cpp: Add option --export-filter.