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.

Fix for PR #3526(Configuration.h LCD & SDCard section rewrite)

It removes duplicated BQ_LCD_SMART_CONTROLLER.

Some clean-up for example configuration files

Follow-up and fix the PR #3253(Make the Autotune item(s) in the LCD menu optional):
・Update forgotten file(Felix/Configuration_DUAL.h)

Follow-up the PR #3526(Configuration.h LCD & SDCard section rewrite):
・Add section of RigidBot Panel to RepRapWorld/Megatronics/Configuration.h

Follow-up the PR #3576(Z Safe Homing for all):
・Adjust spacing

Follow-up and fix the PR #3625(Print job statistics):
・Update forgotten file(Felix/Configuration_DUAL.h)
・Remove ambiguous character "f" in top of RigidBot/Configuration.h

The `Z_SAFE_HOMING` feature is coupled with probes and not leveling, so
make it available for general use.

Following up on #3231

Follow-up the PR #3082
Follow-up the commit 143ad74: unnoticed updates
Fix for commit b0f5ba0: Z offset: -front [of the nozzle] +behind -> Y offset: -front [of the nozzle] +behind
Standardize the writing method: Z-probe, z-probe -> Z Probe
Adjust spacing

documentation/LCDLanguageFont.md moved to http://www.marlinfirmware.org/ at commit 12d0983757,
but http://www.marlinfirmware.org/ isn't working.

follow-up the commit b82d91eca6

Aim: Test probes in update_endstops only when activated

Changes:
Configurations
 Add define for FIX_MOUNTED_PROBE to handle the situation where formerly ENDSTOPS_ONLY_FOR_HOMING had to be set, or lowering the nozzle below Z_PROBE_OFFSET_FROM_EXTRUDER could give an "endstop hit" message.
 Add define for Z_MIN_PROBE_USES_Z_MIN_ENDSTOP_PIN to indicate a common situation, that we have a probe but it is connected to an endstop pin
 Add some comments
 Shift some entries to have related things together.

Conditionals.h
 We have a probe (HAS_Z_MIN_PROBE) if one of the pins is defined AND one of the probes is defined.

SanityCheck.h
 Add some tests if the probe is connected and if we have defined a probe.

stepper.cpp
 Changes to test the probe only when it is deployed (z_probe_is_active).
 Test update_endstops() when the probe is deployed.

MarlinMain.cpp
 a. set and reset z_probe_is_active in deploy_z_probe(), stow_z_probe() and dock_sled()
 b. set and reset z_probe_is_active in the case a z-servo is moved to a defined position. The only remaining unhandled servo move is in M280 where we do not end in a defined position. If you want to handle a probe use M401/402
 c. skip deploying/stowing when already deployed/stowed in the dedicated deploy/stow functions.
 d. Handle the new FIX_MOUNTED_PROBE in parallel to a servo driven probe/endstop.

 To do: In another PR. handle all probes in deploy/stow_z_probe.
 Sort out SERVO_LEVELING vs. HAS_SERVO_ENDSTOPS.

If Marlin is blocking the serial input or command queue for any length
of time (for example more than 2 seconds), it needs to send a message
to serial out to inform the host that it is busy. Marlin should only
send these messages out when busy, and preferably not when trying to
print formatted output.