Remove SF_ARC_FIX from G5 (Cubic B-spline).
SF_ARC_FIX is a fix for G2/G3 (arcs) where Skeinforge always produced relative instead of absolute coordinates for the endpoint of an arc.
It's very unlikely this is also a problem with Cubic B-splines. More likely is copying from the G2/G3 code.

No compromises for the manage_heater(). manage_heater() will return immediately when there is nothing to do, but needs a constant detaT to work proper.
Calling idle() only every 200ms results in a display update every ~2 seconds - that should be enough.
For the other functionalities in idle() and manage_inactivity() 200ms is a lot but hopefully works.

Speedup sreen update delay
and correct a comment.

A module division by 10 is slooooow. (powers of 2 are fast - but then you can AND a bitmask with the same result)

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Minor DELTA segmentation speedup by pulling calculations out of the loop.

Make sure to only set the fan state once when extruders share a fan

Update forgotten Configuration_DUAL.h

Addressing #3743

Idle during long arcs
to prevent from watchdog resets during high segmented fast arcs.

Saves about 2669 bytes when deactivated. (About 1% for a AT2560, about __4%__ for a AT644!)

Add a forgotten condition

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It update forgotten Configuration_DUAL.h.