More message improvements

Minor things, but every little bit helps:

• memory and plot events generate virtual messages in the message window in the generated message group "telemetry/events/XXX". It's a quick and easy way to get a dump of plot and memory events until we get the export feature done.
• virtual messages are colored so they're easy to tell apart from other messages
• virtual messages can be filtered away just like other messages
• user messages (via tmMessage) can be masked off as well with a new checkbox
• the "Warnings" and "Error" checkbox labels are color coded
• "Copy/Save To File" messages now have timestamps prepended

We'll get search/filtering done "soon" once we figure out the ideal UI for it (some people want to search, others want incremental filtering, etc.)