move signal of NativeWindowDestroy to DockWinow

1. improve internal code.
2. move signals of nativeWindowDestroy and qt5ScreenDestroyed from
DockApplet to DockWindow.
3. using QWindow::nativeEvent instead of QAbstractNativeEventFilter

Change-Id: Ic53f2332f51767d42a17188f2eefa2bbd437a3b8