Always pass Core::Id by value.

Currently we pass in some places by value, elsewhere by const ref and
for some weird reason also by const value in a lot of places. The latter
is particularly annoying, as it is also used in interfaces and therefore
forces all implementors to do the same, since leaving the "const" off is
causing compiler warnings with MSVC.

Change-Id: I65b87dc3cce0986b8a55ff6119cb752361027803
Reviewed-by: hjk <hjk121@nokiamail.com>