Implementation of FW extruder change retract

FW retraction is extended onto swap retraction invoked by 'G10 S1'.
Bookkeeping of the retract state of all extruders allows for having one
extruder fw standard retracted while another extruder is swap retracted.
An LCD menu item for the swap retract and recover length was added.
Added thermistor table for the 500C thermistor shipped with the Pico hot end.
Merge pull request #955 from gregington/pico-thermistor

Added thermistor table for the 500C thermistor shipped with the Pico hotend
Fixed stop print LCD function on M104
Inverted state logic for forced_heating_stop variable
Changed the type of variable to bool, the name to "cancel_heatup", flags implementation and added this fix to M190 gcode too.
Merge pull request #965 from bq/forced_heating_stop

Fixed stop print LCD function on M104
Add "Thermal Runaway Protection" feature

This is a feature to protect your printer from burn up in flames if it
has a thermistor coming off place (this happened to a friend of mine
recently and motivated me writing this feature).

The issue: If a thermistor come off, it will read a lower temperature
than actual. The system will turn the heater on forever, burning up the
filament and anything
else around.

After the temperature reaches the target for the first time, this
feature will start measuring for how long the current temperature stays
below the target minus _HYSTERESIS (set_temperature -
THERMAL_RUNAWAY_PROTECTION_HYSTERESIS).

If it stays longer than _PERIOD, it means the thermistor temperature
cannot catch up with the target, so something *may be* wrong. Then, to
be on the safe side, the system will he halt.

Bear in mind the count down will just start AFTER the first time the
thermistor temperature is over the target, so you will have no problem
if your extruder heater takes 2 minutes to hit the target on heating.
Changed the declaration of the variable to Marlin_main.cpp to fix issue on commit #965
Merge pull request #989 from bq/cancel_heatup

Changed the declaration of the variable "cancel_heatup" to Marlin_main.cpp to fix issue on commit #965
Merge pull request #948 from Dim3nsioneer/Marlin_v1

Implementation of FW extruder change retract
Fix for a wrong checksum truncation for certain commands

Positioning of string terminator to truncate checksum from the commands
M23, M28, M30, M32, M928 and M117 was off by one, causing the last
letter of the actual command to be truncated instead of just the
checksum.

In case of the SD commands this caused checksummed commands targeting
existing files to fail since the last letter of the filename was
truncated.

In case of M117 this caused the last given letter not to be displayed.

This patch fixes the off-by-one error and sets the null terminator
on the exact position of the * starting the checksum instead of the
character before that.
Merge pull request #1017 from foosel/fix/checksum_truncation_bug

Fix for a wrong checksum truncation for certain commands