Make dependency of devel/gvfs optional for a several different ports

• this makes it possible for users to not install gvfs by making it an optional dependeny
• and moving it to nautilus directly instead of very high up the gnome stack in libngomeui
• it is on by default in nautilus, so no change for users of that

PR: 228307
Submitted by: VVD <vvd@unislabs.com>