Allow hiding the top-level menu bar on non-macOS OSes

[alexvanyo]

The desktop easel menu bar which contains Easel, Assistant and Export nicely is in the system bar on macOS, which means presenting fullscreen works nicely.

However on Linux, this menu bar is within the window - which means that it isn't possible to go fully immersive fullscreen and hide this menu bar.

Maybe the menu bar should hide when presenting immersively?