OS X 10.8: remove HTML formatting in messages.app for ICQ/AIM

Handlungsanweisung on how to remove the awkward HTML formatting that messages.app on OS X 10.8 (Mountain Lion) produces for ICQ or AIM accounts:

1. quit messages.app
2. open terminal.app
3. if ~/Library/Preferences/com.apple.iChat.AIM.plist.lockfile exists,
   rm ~/Library/Preferences/com.apple.iChat.AIM.plist.lockfile
4. in terminal.app, 
   open ~/Library/Preferences/com.apple.iChat.AIM.plist
5. (XCode will start)
6. Locate ICQ Account under root -> Accounts and set 
   ForceICQPlainText to YES
7. File -> Save
8. Reboot!
   Yes, seriously, boot your box. I know, feels strange on OS X.
   But if you start messages.app before rebooting, the setting above will not be saved. And you have to start over (-> goto 1.) again.

Now, happy plaintext ICQing.