This adds the UTF8_STRING atom and explicit UTF-8 character set MIME
type (text/plain;charset=utf-8) to text content when being sent to the
clipboard under the new multipart support.
This fixes clipboard support under X11 particularly, which generally
looks for the UTF8_STRING atom when looking for text content. This can be
verified with xclip -out -verbose, or trying to do things like paste in
Firefox.
I've noted that there's a number of other older atoms that exist, but
I've refrained from adding them for now. Kitty only seems to set
UTF8_STRING and I've had a hard time finding consensus on what exactly
is the correct set otherwise.
e7c68142e3