Unhandled message type 'MESSAGE_ACK'

[pabs3]

While debugging the crash issue, I noticed that the purple-discord function discord_process_dispatch is not able to handle MESSAGE_ACK messages. I'm not sure about the appropriate response to them, if libpurple has some sort of API for marking messages as delivered, probably purple-discord should call that. Alternately MESSAGE_ACK messages could just be ignored.

PS: I noticed a number of issues in other projects relating to MESSAGE_ACK support, perhaps some of them will indicate the purpose of this message type.

Rapptz/discord.py#1851
discord-net/Discord.Net#84
terminal-discord/weechat-discord#29
Kiskae/DiscordKt#2
discordrb/discordrb#211
discord-jda/JDA#14

(05:31:10) discord: got frame data: {"t":"MESSAGE_ACK","s":322,"op":0,"d":{"version":33647,"message_id":<removed>,"channel_id":<removed>}}
(05:31:10) discord: Unhandled message type 'MESSAGE_ACK'

(06:47:01) discord: got frame data: {"t":"MESSAGE_ACK","s":434,"op":0,"d":{"version":33649,"message_id":<removed>,"channel_id":<removed>}}
(06:47:01) discord: Unhandled message type 'MESSAGE_ACK'