new COPYRIGHT_YEAR macro in clientversion.h

- move COPYRIGHT_YEAR to clientversion.h
-- this allows usage of that information also in other places (e.g. splash
  screen)

- use the new COPYRIGHT_YEAR macro in bitcoin-qt.rc
-- this reduces the places, where we need to change the year further

- fix bitcoin-qt.rc not showing up in Qt Creator 

Intended for #2495 mainly, but allows further usage cases.