Messages sent using /msg should go into the input history of the private message window

[aroben]

When sending a message to someone using /msg, the text you send should be entered into the input history of the new private message window so that you can get back to it using Option-Up

Repro steps

1. Be in a chat room with someone (let's call them xenon)
2. /msg xenon this is the first message i sent you
3. In the new private message window that appears, send another message "this is the second message"
4. Press Option-Up twice

Results

Only "this is the second message" is accessible through Option-Up. You can actually get to your original /msg in the chat room you started in using Option-Up, which is somewhat unexpected. Logically the /msg text is part of the private message, so should be accessible in the private message window (and possibly only in that window).

[akempgen]

related to #827