A Click on an empty space in the window overview shows the desktop. That means: It minimizes all windows on the current workspace.
Note: Binary files aren't shown on the web site. To see all files, please download the extension zipfile.
Mutter has this feature natively (meta_screen_show_desktop()), if you want you can ask to make it public for 3.6.1 or 3.8.
Who can I ask to make meta_screen_show_desktop() a public function? https://bugzilla.gnome.org/ ? Would be easier indeed.
Yes, bug for mutter. Thank you!
Show Desktop from Overview