if the past we had lots of crashes because of unexpected unwrap failures,
mostly related to string.
this commit avoids them eg. for string-conversions that may panic
eg. when encountering a null-byte or by logical programming errors
where an object is assumed to be set but is not under unexpected circumstances.
* 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