Add support for importing video stickers

[MSDarwish2000]

This pull request contains a few code changes to enable video stickers importing.

Currently, though documentation of Importing Stickers to Telegram mentions that it is possible to import video stickers the Android client cannot handle them. I've tried to report this issue through e-mail with no response, so decided to try and make the required changes. Hope this finds its way to the developers.

Changes

• updating "TL_stickers_createStickerSet"
• detecting WebM magic bytes
• adding necessary isVideo fields to certain models
• changes in "StickerEmojiCell" for previewing WebM to work

What needs to be reviewed

• Whether it is necessary to validate WebM first? If yes, local or remote?
• code changes made in "StickerEmojiCell"

[MSDarwish2000]

@xaxtix @DrKLO

[stayf]

@xaxtix @DrKLO Добрый день. Слейте пожалуйста этот MR, если он корректный. Давно уже жду поддержи видеостикеров для сторонних приложений под андроид. В Ios поддержка со дня релиза видеостикеров, андроид отстает.

[Mr-lidajun]

这是来自我的假期自动回复邮件。您好，我最近正在“夏威夷”，无法亲自回复您的邮件。我将在假期结束后，尽快给您回复。