Make the window close buttons do the right thing

[sebjameswml]

Toggle blur/offset windows; close app if it's the main window.

[sebjameswml]

I added code in b2949ae which should do the right thing, but cv::WND_PROP_VISIBLE doesn't appear to work on Ubuntu 18. Perhaps it would work on Ubuntu 20; I'll see.

[sebjameswml]

Nope, same problem on the version of OpenCV on Ubuntu 20.04.