Fix fullscreen window position (#873)

Created by: spasche

From my testing on a Fedora 25 (Gnome 3.22) system, it looks like placing the window in fullscreen from the idle callback is too soon and always places the window on monitor 0 (left one). If gtk_window_fullscreen() is called from the "map-event" event, it places the window on the same monitor where the window was. I didn't experience the black border issue mentioned in the previous comment. Using a signal instead of the idle callback means we don't need to track the event source identifier, which removes some code.

I'd be happy if somebody could test this on other environments (non Gnome, or pre-3.22 versions).