Selection: move commands/command_private.* to core/selection.*

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>

core/CMakeLists.txt

@@ -139,6 +139,8 @@ set(SUBSURFACE_CORE_LIB_SRCS
 	save-html.h
 	save-profiledata.c
 	save-xml.c
+	selection.cpp
+	selection.h
 	sha1.c
 	sha1.h
 	ssrf.h