Improve control over which file manager manages the desktop.

This adds a new show-desktop-icons gsettings key to Nemo's preferences, along with adding control over the traditional key that Nautilus uses.  

Nautilus has an autostart condition that checks for schema org.gnome.desktop.background, key show-desktop-icons being true.  If we move Nemo's check to a different key, we can then reliably control which file manager autostarts and provides the desktop.

If Nautilus is not installed, the 'prevent Nautilus' option is not displayed, to prevent confusion.

This could partially address #38.