core: convert parse-xml.c and parse.c to C++

This was very annoying, because the old code was not const-clean
at all and trampled all over buffers. This makes the new code
pretty messy for now.

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

core/CMakeLists.txt

@@ -134,8 +134,8 @@ set(SUBSURFACE_CORE_LIB_SRCS
 	ostctools.c
 	owning_ptrs.h
 	parse-gpx.cpp
-	parse-xml.c
-	parse.c
+	parse-xml.cpp
+	parse.cpp
 	parse.h
 	picture.c
 	picture.h