callwidget: modify call state change behavior

callwidget.cpp

conversationitemdelegate.cpp

distantwidget.cpp

distantwidget.h

mainwindow.cpp

previewwidget.cpp

previewwidget.h

rendermanager.cpp

smartlistmodel.cpp

smartlistmodel.h

videooverlay.cpp

videooverlay.h

videoview.cpp

videoview.h

videowidgetbase.cpp

videowidgetbase.h