add sticker type (#653)

* add sticker type

this pr adds the message type 'sticker'.
stickers are handled as normal images
but tagged with the header `Chat-Content: sticker`
it's up to the ui to render these stickers appropriate.

* cargo fmt

src/chat.rs

@@ -646,6 +646,7 @@ pub fn msgtype_has_file(msgtype: Viewtype) -> bool {
     match msgtype {
         Viewtype::Image => true,
         Viewtype::Gif => true,
+        Viewtype::Sticker => true,
         Viewtype::Audio => true,
         Viewtype::Voice => true,
         Viewtype::Video => true,