libavg_app: added hook facilitating features additions on subclassing Apps
libavg_app: added hook facilitating features additions on subclassing Apps

svn path=/branches/libavg_app/; revision=8976

yyd01245/libavg,hoodie/libavg,yyd01245/libavg,hoodie/libavg,scotty007/libavg,yyd01245/libavg,hoodie/libavg,libavg/libavg,libavg/libavg,scotty007/libavg,th3infinity/libavg,scotty007/libavg,libavg/libavg,hoodie/libavg,libavg/libavg,yyd01245/libavg,th3infinity/libavg,scotty007/libavg,th3infinity/libavg,th3infinity/libavg