Because of the way data virtualization works (or doesn't) in WPF,
this was a whole lot different from WinForms. What I'm doing is
pretty simple, so I avoided the complexity (and quirks) inherent
to more complete data virtualization solutions. (All I really want
is to not render the entire thing up front.)
I haven't figured out how to make the AppendText(text, color) extension
work right with the WPF version of RichTextBox, but I was able to work
around the issue, and it's not really worth sinking time into.
