PrinterMain: Move heater safe temperature checking from PWM timer ISR to control calculation event and to blinker.
This will allow using hardware PWM for heaters. It's even generally safer since the blinker event feeds the watchdog. Though the response time is a bit worse.
PrinterMain: Take invert into account for heater and fan emergency action.
Refactoring to make heater and fan PWM abstract.
This will allow writing the final glue needed to use hardware PWM.