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