Problem: Unable to tell whether a msg_show event is emitted as a result
a command typed on the cmdline (UI may want to represent these
differently from other messages).
Solution: Add trigger parameter that is set to "typed_cmd" for
a message emitted due to an interactively typed command.
Possible extensions are mapping/timer/event but it's hard to
imagine a UI distinguishing those so not added here.
97549ad7cf