Optional support to store text strings in Flash

RAM likely won't run short on this project, but a macro has been added
wrapping all text, which as an example can be used as the Arduino F(x)
macro.

Also changed idleTemp to a defined value; this saved ~50bytes of Flash.

TFT_ defines are now LCD_ defines.

Need to look into why space is a consistent ~70 bytes higher when using
PROGMEM; the lack of variables in .bss should reduce this size, should
it not?
Fixed garbage when editing digits of variable size

If the amount of digits changed when editing something, such as fan
speed, garbage would be left behind. Smallest hack I could find just
prints some spaces at the end.