Fix a regression when updating attributes

This fixes a regression introduced in 3a062e6a and 6054e42d where some
attributes were not sent to the Akonadi server in update job even though
they were modified. This was due to a bad API design which returns a
non-const pointer to an attribute from a const method, so callers sometimes
modify the returned attribute on a const object. Since the method itself
is const though, it did not mark the attribute as modified.

Proper fix is to introduce a purely const and non-const overloads for
the attribute accessors, unfortunatelly this requires fixing a lot of our code
in many places first to not abuse the non-constness of the returned
attribute.

Note that since the code is in an inlined method, all clients should be
recompiled.

CCMAIL: faure@kde.org