Opens/closes the friends pane.
- tabIndex optional - picks a specific tab in the friends frame
- 1 = friends tab
- 2 = who tab
- 3 = guild tab
- 4 = raid tab
- default = last open tab
- If the friends frame was already open, this closes the frame.
- If it was not already open, it is opened at the guild tab.
- Opens/closes the friends pane.