Add option to show/hide desktop icons altogether

Signed-off-by: Niklas Koep <niklas.koep@gmail.com>
Merge branch 'show-desktop-icons-option' of https://github.com/nkoep/Cinnamon into nkoep-show-desktop-icons-option
Merge branch 'show-desktop-icons-option' of https://github.com/nkoep/Cinnamon
Improved alignment of items in cinnamon-settings desktop module
Merge branch 'nkoep-show-desktop-icons-option'