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 needed to be able to re-initialize the custom characters in response to a menu change, so I added hooks for that to ultralcd code and the hitachi lcd code. New custom characters may be added to enhance other sub-menus by modifying this hook.
I noticed in passing that this can replace all the lines above.