Follow-up to #4666

Also improve the documentation of this and related options, and set
EXTRUDE_MINTEMP to 170 everywhere.

- `per_sq_second` => `per_s2`
- `per_sqr_second` => `per_s2`
- `axis_steps_per_sqr_second` => `max_acceleration_steps_per_s2`

This is an update of MarlinDev PR #196.

G20/21: support for switching input units between millimeters and
inches.
M149: support for changing input temperature units.

In support of these changes, code_value() and code_value_short() are
replaced with an array of functions which handle converting to the
proper types and/or units.

Follow-up the PR #3643(Temperature singleton)

・Change from fanSpeedSoftPwm[0] to thermalManager.fanSpeedSoftPwm[0] in planner.cpp
It fix compilation error when FAN_SOFT_PWM is enabled.

・Remove declaration of setExtruderAutoFanState() in temperature.h
Because that function was abolished.

・Change from babystepsTodo to thermalManager.babystepsTodo in ultralcd.cpp
It fix compilation errors when BABYSTEPPING is enabled.

- Search all symbols and apply prefixes where needed
- Encapsulate some private methods
- Inline some setters
- Make `microstep_mode` a public method