Change QConnectionSenderSwitcher et al to use signal index range

Change QConnectionSenderSwitcher et al to use signal index range

First step towards getting rid of the signal_absolute_index variable
from QMetaObject::activate() (which requires computation of the
class's method offset).

This also required changing the implementation of the public function
senderSignalIndex() so it still returns an index in the full method
range.

Change-Id: I58571eb3c8099ea5b673682872c53875f5ea8c13
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>