per-group show/hide avatar permission

The current phpBB3 release (3.0.7-PL1 at the time of writing this) allows to show/hide avatars on a per-user basis.

However, there're some possible uses of phpBB3 where the display of avatars (when navigating topics and visiting user profiles) would be desirable to be enabled/disabled on a per-group basis.

Imagine there's a community where users upload their real photo as their avatar, but they want that only trusted users are able to see their photo. This would be hard (or nearly impossible) to maintain user-by-user, but would be very convenient doing it by groups.

A board that I just created needs this feature. I implemented it as a temporary hack (using the "can send emails" permission as also "can see avatars" permission), but I think it would be great to have this in future phpBB releases, so that I don't have to re-hack the code when I upgrade.