Change copyright from Intevation to BSI
* src/Makefile.am, src/addin-options.cpp, src/addin-options.h, src/application-events.cpp, src/attachment.cpp, src/attachment.h, src/common_indep.c, src/common_indep.h, src/eventsink.h, src/eventsinks.h, src/exechelp.c, src/exechelp.h, src/explorer-events.cpp, src/explorers-events.cpp, src/gpgoladdin.cpp, src/gpgoladdin.h, src/gpgolstr.cpp, src/gpgolstr.h, src/icons/Makefile.am, src/mail.cpp, src/mail.h, src/mailitem-events.cpp, src/mimedataprovider.cpp, src/mimedataprovider.h, src/mimemaker.cpp, src/mlang-charset.cpp, src/mlang-charset.h, src/mymapi.h, src/oomhelp.cpp, src/oomhelp.h, src/parsecontroller.cpp, src/parsecontroller.h, src/rfc2047parse.c, src/rfc2047parse.h, src/ribbon-callbacks.cpp, src/ribbon-callbacks.h, src/windowmessages.cpp, src/windowmessages.h, tests/Makefile.am, tests/run-parser.cpp, tests/t-parser.cpp: Change copyright to BSI.
This should make it more transparent who the copyright holder
actually is as most work since 2015 was done under development
contracts.