Add internal functions to QObjectPrivate for signal & slot connections in QML

In QML it is common to connect signals to slots that are implemented as
JavaScript functions. QML used to maintain separate data structures that
mirrored the QObject connection list and kept references to the JavaScript
objects necessary to perform the call on signal activation.

The recent addition of functor based QObject::connect makes it possible
to store this information in QSlotObjectBase sub-class instead, which
eliminates any extra bookkeeping.

This patch adds internal connect and disconnect overloads to QObjectPrivate
that allow for connecting QObject *sender, int signalIndex to a given
QSlotObjectBase and similar for disconnect.

Change-Id: I90f43d13eb95bd884d752484cf4faacc446f4d6a
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>