Qt documentation recommends calling saveGeometry() in closeEvent().
This commit fixes the following bug on my GNU/Linux with Xfce system:
1. Move the top of the YACReader window to the top of the screen.
2. Restart YACReader (exit and run again).
2. Enter full screen mode.
4. Restart YACReader.
5. Exit full screen mode.
At this point YACReader's title bar is hidden beyond the top of the
screen, i.e. the window has moved up.
closeEvent is accepted by default, so this commit does not change the
application behavior. But Qt documentation recommends not relying on the
default value as subclasses may choose to clear it in their constructor.
signal is emitted so it is actually processed and we don't get dangling pointers.
Use modern signal slot syntax so that the compiler and source check tools can
verify we actually fixed the problem.
