Consider acceleration to estimate timings

[DrLex0]

Add an option to take acceleration into account when calculating the duration of print moves. This will make the timing of the fan speed changes more accurate. Currently the timings can be off by quite a bit if there are many small zig-zag moves right before a fan speed change, because these are slowed down considerably by accelerated moves.
Ideally the splitting of print moves should also take acceleration into account, but I reckon this will require a lot of effort for only a marginal improvement, the simple linear split should be good enough.