Animated Titles: Use QPlainTextEdit for multi-line

[ferdnyc]

Qt offers QPlainTextEdit which is supposed to be slightly faster in some situations than QTextEdit, and since we're always explicitly accessing the content as plain text, it just makes sense.

(Also, another "while I was in there": The QBlenderEvent class was entirely unused, so I yanked it.)

[jonoomph]

I could have sworn we use QBlenderEvent, lol. It was tricky to get our own thread to sent UI events back to the Blender dialog... which is what this relates to. But perhaps we implemented that a different way in the end, and this became a left over?

[jonoomph]

👍