cleanup: fold core/divecomputer.cpp into core/device.c

core/device.h was declaring a number of functions that were related
to divecomputers (dcs): creating a fake dc for manually entered dives
and registering / accessing dc nicknames. On could argue whether
these should be lumped together, but it is what it is.

However, part of that was implemented in C++/Qt code in a separate
core/divecomputer.cpp file. Some function therein where only
accessible to C++ and declared in core/divecomputer.h.

All in all, a big mess. Let's simply combine the files and
conditionally compile the C++-only functions depending on
the __cplusplus define.

Yes, that means turning device.c into device.cpp. A brave soul
might turn the C++/Qt code into C code if they whish later on.

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

profile-widget/profilewidget2.cpp

@@ -1,7 +1,6 @@
 // SPDX-License-Identifier: GPL-2.0
 #include "profile-widget/profilewidget2.h"
 #include "qt-models/diveplotdatamodel.h"
-#include "core/divecomputer.h"
 #include "core/subsurface-string.h"
 #include "core/qthelper.h"
 #include "core/picture.h"