Ctrl-W should close viewer instead of main app

[wzhliang]

Marta version 0.8.1

1. Focus on an picture (say png file)
2. Press SPACE to view it
3. Press Command-W

This currently closes the main app and leaves the viewer alone. In my opinion, the opposite should happen.

[page-down]

Currently you can press Space again to close it.

For macOS, however, I think Command + W should be used to close Quick Look, not Ctrl + W, refer to the behavior in Finder.

[wzhliang]

Correction. I was using Command + W. At the mean time, I'll try to get used to Space