Implement AVR Platform

Addressing #6992 and #5851

* Remove ADVANCED_PAUSE_FEATURE requirement for PROBING_HEATERS_OFF

* Add HEATER_IDLE_HANDLER conditional

Removing redundant_temperature from public section and leaving it in the
private section

* Unify M600 and M125 pause features
* Cleanup per thinkyhead's comments
* Rename filament_change_menu_response to advanced_pause_menu_response
* Include HAS_BED_PROBE in QUIET_PROBING
* Update gMax example file
* is_idle() is out of scope without the braces
* Convert FT-i3-2020 to Advance Pause names...
* Allow pause even if not printing

Saves a few lines of code.

 - rename to PROBING_HEATERS_OFF
 - move heater pausing functionality into thermalManager
   - add variables, pause(), ispaused(), other functions
 - add fan pausing functionality -> PROBING_FANS_OFF
 - add probing_pause() wrapper
 - move pausing into do_homing_move() and do_probe_move() to minimize quiet time and so other probe types can benefit
 - example configs

spi.h is only needed if HEATER_0_USES_MAX6675 is defined, so making its inclusion conditional on HEATER_0_USES_MAX6675 being defined.

After wraparound, pwm_count <= pwm_mask holds, thus soft_pwm_X <= pwm_count
guarantees soft_pwm_X < pwm_mask is true, and the heater will be switched
off in the first branch.
Do not evaluate the pwm conditions a second time, this reduces the
instruction count (4 instructions per PWM) and text size (6 byte).

Signed-off-by: Stefan Brüns <stefan.bruens@rwth-aachen.de>

The compiler is not able to reuse the value of pwm_count, but reloads it
on every evaluation, if is stored in a static variable, as it cannot prove
it will be unchanged. A variable with local scope may not be modified from
the outside, so its value can be reused.
Doing so reduces text size and instruction count.

Signed-off-by: Stefan Brüns <stefan.bruens@rwth-aachen.de>

If dithering is enabled, the remainder of the soft_pwm_X duty value at
turnoff time is added to the next cycle. If e.g. the duty is set to 9 and
SCALE is set to 2, the PWM will be active for 8 counts for 3 cycles and
12 counts on each fourth cycle, i.e. the average is 9 cycles.

This compensates the resolution loss at higher scales and allows running
fans with SOFT_PWM with significantly reduced noise.

Signed-off-by: Stefan Brüns <stefan.bruens@rwth-aachen.de>

A 128 step PWM has 127 intervals (0/127 ... 127/127 duty). Currently, a
PWM setting of 1/127 is active for 2/128, i.e. double the expected time,
or, in general n+1/128 instead of n/127.
Fixes issue#6003.

Signed-off-by: Stefan Brüns <stefan.bruens@rwth-aachen.de>

If Marlin is inside the temperature ISR, the stepper ISR is enabled. If
a stepper event is now happening Marlin will proceed with the stepper
ISR. Now, at the end of the stepper ISR, the temperatre ISR gets enabled
again. While Marlin proceed the rest of the temperature ISR, it's now
vulnerable to a second ISR call.