Port QBuffer to QMetaMethod-based connectNotify()

Port QBuffer to QMetaMethod-based connectNotify()

The const char *-based API is deprecated and will be removed in Qt5.

Also fix a bug in the disconnectNotify() reimplementation; when all
signals are disconnected at once, disconnectNotify() is only called
a single time, with an invalid method as argument. Thus, the signal
connection count should be set to 0, instead of decremented.

Change-Id: Ieee92293777bff87f8b28e56e23ab55d0b8b8101
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>