stickers: add name of sticker pack in alt text

[pacien]

Currently, stickers bridged Telegram->Matrix contain the emoji and its textual
description as alternative text body.

It would be nice to also include the name of the sticker pack in this alt text.

This would be especially convenient in text-only terminal-based clients for
example. (I don't like the screen estate that stickers are taking on my screen,
but I still like to know what the stickers are quickly without having to open
them).

Current events are like (other fields omitted):

{
  "type": "m.sticker",
  "content": {
    "body": "😔 (pensive face)"
  }
}

This would instead be something like so, for a sticker pack named "Funny cats":

{
  "type": "m.sticker",
  "content": {
    "body": "Funny cats: 😔 (pensive face)"
  }
}