Extend .GM visibility command

Extend .GM visibility command

Extend the .GM visibility command.

Currently only options are:

.gm visibility on
.gm visibility off

I suggest adding the following:

.gm visibility group

This will be useful for situations where we need to login to help grouped players in busy zones that are bugged on a quest for example.
This option would make it so only those in the group or raid group could see/target the game master.