core: make owning pointers a top-level features

The undo-code uses owning pointers based on std::unique_ptr to
manage lifetime of C-objects. Since these are generally useful,
move them from the undo-code to the core-code. In fact, this
eliminates one instance of code duplication.

Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>

core/CMakeLists.txt

@@ -132,6 +132,7 @@ set(SUBSURFACE_CORE_LIB_SRCS
 	metrics.cpp
 	metrics.h
 	ostctools.c
+	owning_ptrs.h
 	parse-gpx.cpp
 	parse-xml.c
 	parse.c