Support listening/watching activity types

This resolves #931

As part of this change, StreamingType has been refactored to realign
with how Discord seems to define the 'type' field on activities now.

StreamType is renamed to ActivityType, and the following properties have
been changed:
- NotStreaming -> Playing
- Twitch -> Streaming

Additionally, the StreamType property/parameter has been removed from
StreamingGame, and moved up a scope to Game.

Normal Games may now set their type, to line up with changes in
Discord's official clients.

src/Discord.Net.Rest/API/Common/Game.cs

@@ -12,7 +12,7 @@ namespace Discord.API
         [JsonProperty("url")]
         public Optional<string> StreamUrl { get; set; }
         [JsonProperty("type")]
-        public Optional<StreamType?> StreamType { get; set; }
+        public Optional<ActivityType?> Type { get; set; }
         [JsonProperty("details")]
         public Optional<string> Details { get; set; }
         [JsonProperty("state")]