Avoid accessing QObjectPrivate::currentSender directly from QtDBus

Avoid accessing QObjectPrivate::currentSender directly from QtDBus

In particular, currentSender->signal is about to be changed from
the method index range to the signal index range (i.e., excluding
non-signal methods). The public senderSignalIndex() function will
still return the index in the method range, so QtDBus will then
continue to work without change. If necessary, QtDBus can afterwards
be ported back to using the internal API again (including the new
QMetaObjectPrivate API for working with methods in the signal range).

Change-Id: I58212e859560a8e241adee99dd2da9ef009d4e5b
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>