HistoryWindow - distinguish read from unread text

[SebastianJay]

Within HistoryWindow, we should have a way to differentiate between text that just came into the window from a recent action (unread) and text that is from a previous action (read) -- this is particularly useful for people who want to skip text animations to see where they left off.

One strategy is to gray out the old text. Within the draw method, keep track of an index (likely a row index) that marks the edge of the read text, and use color formatting to set the read text to a readable but distinct color. The formatting list will need to ignore other color/style commands that are contained within read text.