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.
abb40ecedd
