Add hovering slideshow toolbar, Save window states to config file

Hovering toolbar

When a slideshow is running or when the mouse approaches the top of the window and the controls are hidden, a toolbar will gradually slide out. The toolbar allows the slideshow to be customized and provides a button to show all controls if they are not visible.

koko-hoveringtoolbar-2021-09-11_09.42.36

Window state saving

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.

---

@carlschwan, @mikeljohnson, @teams/vdg