Automatically mark scrollback when the log is in background.

Opening this Pull Request mostly for discussion.

I didn't see that having to manually mark the scrollback lines made much sense. To me it makes much more sense for this to automatically get set every time the log is unselected, or the whole window loses focus.

Also, curious what people think about this being an "always on" feature. I tend to be the person that avoids adding preferences if I can, but I don't feel super strongly about this one.