Draft: Add keybinding to toggle the menubar

Currently there is no way to hide the menubar. This merge request adds a keybinding for toggling display of the menu bar. I chose Alt+m and F12 as default. Feel free to come up with other suggestions what binding should be used here. Also, I'm not sure how this bar is called. It's not a toolbar, so a menu bar???

This partially fixes #1202.

I guess a note about this should also be added to the manual.