.) Use already existing inverse_millimeters instead of /
block->millimeters.
.) Prevent overflow during acceleration calculation by checking if float
is necessary. Idea modified from Sailfish.
.) Save two uint32_t or even float multiplications by checking if
step[AXIS] has steps and if max acceleration is lower than accel. If
not, there is no need to check this axis.

MSG_INFO_PRINT_TIME is still being used

As pointed out by #5044

As requested in #5031

We don't want to have retract/prime movements done with LIN_ADVANCE, that's also true with simultaneously executed Z-hops.

With this formula, a start value for K for further fine-calibration can be calculated.

As noted by https://github.com/MarlinFirmware/Marlin/pull/4971#issuecomment-252503295

Made Filament change Text sound like correct german.

・Add forgotten '\'

…and apply const to some method parameters

This breaks the raw option, but in the future perhaps all planner moves
will be sent in raw form instead of logical.