The file command_private.cpp had functions concerning selections
only. To make these functions accessible from outside the undo
machinery, turn it into a part of the core-library. Currently,
only C++ functions are exported. We might think about also
exporting a C interface.
Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>
In the future we might want to use undo-commands for mobile as
well (even if not implementing undo).
Therefore, move the undo-command source from desktop-widgets
to their own commands top-level folder.
Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>
