preliminiary implementation for the early heating finish.

Might be replaced by something more clever, e.g. by erik, and does not yet support the second extruder or the bed.
its kind of not so cool, because you need 6 more ints.
Maybe isheating() should use the degrees directly, as it is not used in time-critical anyways.
Then it would be much easier. to have the offsets without additional variables.

Marlin/Configuration.h

@@ -91,6 +91,8 @@
 // if CooldownNoWait is defined M109 will not wait for the cooldown to finish
 #define CooldownNoWait true
 
+// Heating is finished if a temperature close to this degree shift is reached
+#define HEATING_EARLY_FINISH_DEG_OFFSET 1 //Degree
 // PID settings:
 // Uncomment the following line to enable PID support.