feat: hide all windows (show desktop) via Super+D

[salim-b]

Upstream issue proposing this (years ago) is found here: https://gitlab.gnome.org/GNOME/gsettings-desktop-schemas/-/issues/24

Justification to not set the shortcut by default by GNOME dev (Ubuntu has that default) is:

The original purpose of that shortcut was to provide quick access to desktop icons (in fact, the internal shortcut name is show-desktop). As we no longer have those, the shortcut is no longer useful to most users.

Main counter-argument is: Being able to hide all windows (and showing one of Bluefin's beautiful wallpapers 🦕 or whatever) is already a use case.

GNOME dev counters that with

Sure, and nobody is planning to remove the shortcut. But it's not a strong argument for a default shortcut, given that someone who installs and configures conky can also assign a shortcut.

Now I just wanted to see if anyone is against setting this by default in Bluefin.

[castrojo]

Bringing the classic Ubuntu desktop is what we do around here, appreciate this one!