Add option to hide the active window highlight border when there is only one window on the desktop

[jwilger]

I love to have the active window highlight on when I have multiple windows on the screen in tiling mode, but when I only have one window taking up the whole screen, the active window highlight is off-putting. (If anyone knows of a way to do this that isn't exposed via the settings UI, please do let me know! 🙏🏻 )