LCD Progress Bar

Display a graphical progress bar on the bottom line of the LCD during SD printing if there's no other LCD status message. If there is a message, display it in alternation with the progress bar, about every 2 seconds with a normal lcd update rate.

This feature makes it easier to gauge print progress from across the room, or at a glance.

To gain this feature we sacrifice three other custom characters, substituting \xDF for the "degree" character, infinity for the "refresh" character, and caret (^) for the "uplevel" character.
Minor performance improvement for the status screen.