feat(ui): "append" parameter for "msg_show" UI events

Problem:  Consecutive "msg_show" events stemming from an `:echon`
          command are supposed to be appended without a newline, this
          information is not encoded in the "msg_show" event.
Solution: Add an "append" parameter to the "msg_show" event that is set
          to true to indicate the message should not start on a new line.
Considered alternative: Emit a newline for the common case instead at the
start of a new message. That way UIs can more closely follow the logic
as it is implemented for the message grid currently. This would be a
breaking change. The "append" parameter seems OK.

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