Added ability to toggle on/off the small bottom-left menu

[rpapallas]

Sometimes, especially for experiments, it's good to have a clean window without any menus to take screenshots. I added a small code that provides a toggle to turn on or off the bottom-left stats menu. I also added an optional parameter to turn it off when initializing the viewer. By default, nothing changes, the stats menu will be visible as like before.

I had to introduce two different names for these menus: help_menu for the previous menu and statistics_menu for the bottom-left one.

[rohanpsingh]

Hi, thanks for the PR. I think it is useful to be able to hide/unhide the stats menu.
But it might be better to simply clumb this toggle on/off functionality with the main "help menu", rather than creating a new keyboard binding.

[rpapallas]

Hi @rohanpsingh just pushed the new changes. Let me know if this is better. Thanks for sharing this library, it's really useful.

[rohanpsingh]

Thanks, I'll merge!