Shrink the window to match letterboxing size when the emtpy area is doble-clicked

Now that users can reuse last window size (#41918 (closed)) and the option is exposed in the UI (#41916 (closed)), ending with a "permanently letterboxed" window becomes easier, as noted by @thorin.

Hence I propose an easy and intuitive way for resizing the window down to letterboxing size (hence removing the margin), by just clicking the empty letterboxing around the content.

This feature can be signaled by showing a zoom-out cursor when hovering that area (screenshot below).