Cleanup: Move *_loc formatting functions into new format.cpp file

qthelper.cpp is already quite voluminous. Move the recently
introduced localized versions of (v)snprintf() and put_format()
into their own translation unit.

Moreover, adopt C-style semantics for asprintf_loc(). This function
will be used to remove fixed-size buffers in core/plannernotes.c.

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

core/CMakeLists.txt

@@ -77,6 +77,7 @@ set(SUBSURFACE_CORE_LIB_SRCS
 	# dirk ported some core functionality to c++.
 	qthelper.cpp
 	metadata.cpp
+	format.cpp
 	divecomputer.cpp
 	exif.cpp
 	subsurfacesysinfo.cpp