Add fullscreen mode

Check https://developer.gnome.org/hig/stable/fullscreen.html.en Also https://blogs.gnome.org/alexm/2020/04/12/on-windows-and-titlebars/

See what Boxes and Vinagre do.

Some Boxes users aren't satisfied with the topbar behavior, since it expands horizontally and can accidental steal cursor events from the topbar of the display OS (if it is GNOME or something similar with hot-corners on top). I suggest we create a centralized and small widget, like browsers do. We shouldn't offer many options in the fullscreen topbar, other than Unfullscreen.