Cleanup: fold core/helpers.h into core/qthelper.h

helpers.h included qthelper.h and all functions declared in helpers.h
were defined in qthelper.h. Therefore fold the former into the latter,
since the split seems completely arbitrary.

While doing so, change the return-type of get_dc_nichname from
"const QString" to "QString".

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

packaging/ios/Subsurface-mobile.pro

@@ -133,7 +133,6 @@ HEADERS += \
 	../../core/dive.h \
 	../../core/git-access.h \
 	../../core/gpslocation.h \
-	../../core/helpers.h \
 	../../core/imagedownloader.h \
 	../../core/pref.h \
 	../../core/profile.h \