Fixing some gaps with nested emote chains and EmoteManager.IsBusy

[gmriggs]

Repro steps:

1. Start the cow tipping quest by speaking with Dwennon in Holtburg. If you've already completed the cow tipping quest, you can erase it with /qst erase cowtip beforehand.
2. Level up your cowtipcounter quest to 300 tips. If you need a quick function to do this:

        [CommandHandler("cow", AccessLevel.Developer, CommandHandlerFlag.RequiresWorld)]
        public static void HandleCow(Session session, params string[] parameters)
        {
            for (var i = 0; i < 300; i++)
                session.Player.QuestManager.Stamp("cowtipcounter");
        }

3. Approach Dwennon, and then repeatedly spam the R key on him

Expected:
The 'award' emote chain only happens once

Actual:
The 'award' emote chain happens multiple times

Resolution:
EmoteManager.Enqueue() was not taking nested emote chains into account properly for setting IsBusy=false. Now the Nested level is tracked properly, and IsBusy is only set to false at the very end of the emote chain.