API inconsistency between `AvatarGroup.bindItems` and `MessageList.bindItems`

[sissbruecker]

Both AvatarGroup and MessageList have a bindItems method for binding their items to a list signal. However the return type of the SignalBinding differs between them:

// MessageList
SignalBinding<List<Signal<MessageListItem>>> bindItems(Signal<List<Signal<MessageListItem>>> signal);

// AvatarGroup
SignalBinding<Collection<AvatarGroupItem>> bindItems(Signal<List<Signal<AvatarGroupItem>>> signal);

Note that:

• the signal binding returned from MessageList.bindItems has a list of signals of message list items
• the signal binding returned from AvatarGroup.bindItems has a collection of avatar items

This should probably be aligned so that AvatarGroup returns a signal binding with a list of signals.