Add skull glyph and highest character value const

Add a constant containing the highest character defined in our font,
and add a skull glyph at value 127 (DEL) for fun.
Cache message in RAM. Protect against overrun.

Load the message into RAM on startup. Use the same load routine after
a new message is defined in the serial handler.

Add a stub for a future demo routine, which will be called if either
the message length is zero, or it is 255 (which is what EEPROM is set
to when erased).

In order to detect uninitialized EEPROM, our max message length is
limited to 254 characters.
put local loop var inside for delcaration
global var cleanup. misc fixes.
implement some demo functions
add ctrl-c handler to serialEvent
fix demo loop off-by-one
fix small mistake in demo loop
add alternating lines demo mode. slow all demos.
ifdef out serial code. seems to flake out very easilly
update tool to match serial code
reduce scroll refresh count expecting clock reduction