Save window states to config file

Whenever Koko is closed, enters fullscreen mode or starts a slideshow,
save the window state.

Whenever Koko is opened, leaves fullscreen mode or stops a slideshow,
restore the window state.

I'm not sure if it's an X11 bug or a Qt Quick bug, but you can't restore
from fullscreen to maximized. Exiting fullscreen mode will always go to
the windowed state.