Shift call of start_watching_heater() into setTargetHotend()

to warrant watching is set or reset.

Make setTargetBed() in _lcd_preheat() dependant of TEMP_SENSOR_BED.
Use disable_all_heaters() in lcd_cooldown() and abort_on_endstop_hit.

Marlin/stepper.cpp

@@ -296,11 +296,7 @@ void checkHitEndstops() {
         card.sdprinting = false;
         card.closefile();
         quickStop();
-        setTargetHotend0(0);
-        setTargetHotend1(0);
-        setTargetHotend2(0);
-        setTargetHotend3(0);
-        setTargetBed(0);
+        disable_all_heaters(); // switch off all heaters.
       }
     #endif
   }

Marlin/temperature.h

@@ -96,7 +96,12 @@ FORCE_INLINE float degBed() { return current_temperature_bed; }
 FORCE_INLINE float degTargetHotend(uint8_t extruder) { return target_temperature[extruder]; }
 FORCE_INLINE float degTargetBed() { return target_temperature_bed; }
 
-FORCE_INLINE void setTargetHotend(const float &celsius, uint8_t extruder) { target_temperature[extruder] = celsius; }
+FORCE_INLINE void setTargetHotend(const float &celsius, uint8_t extruder) {
+  target_temperature[extruder] = celsius;
+  #ifdef WATCH_TEMP_PERIOD
+    start_watching_heater(extruder);
+  #endif
+}
 FORCE_INLINE void setTargetBed(const float &celsius) { target_temperature_bed = celsius; }
 
 FORCE_INLINE bool isHeatingHotend(uint8_t extruder) { return target_temperature[extruder] > current_temperature[extruder]; }

Marlin/ultralcd.cpp

@@ -515,12 +515,11 @@ static void lcd_tune_menu() {
 
 void _lcd_preheat(int endnum, const float temph, const float tempb, const int fan) {
   if (temph > 0) setTargetHotend(temph, endnum);
-  setTargetBed(tempb);
+  #if TEMP_SENSOR_BED != 0
+    setTargetBed(tempb);
+  #endif
   fanSpeed = fan;
   lcd_return_to_status();
-  #ifdef WATCH_TEMP_PERIOD
-    if (endnum >= 0) start_watching_heater(endnum);
-  #endif
 }
 void lcd_preheat_pla0() { _lcd_preheat(0, plaPreheatHotendTemp, plaPreheatHPBTemp, plaPreheatFanSpeed); }
 void lcd_preheat_abs0() { _lcd_preheat(0, absPreheatHotendTemp, absPreheatHPBTemp, absPreheatFanSpeed); }
@@ -602,11 +601,7 @@ void lcd_preheat_abs0() { _lcd_preheat(0, absPreheatHotendTemp, absPreheatHPBTem
 #endif // more than one temperature sensor present
 
 void lcd_cooldown() {
-  setTargetHotend0(0);
-  setTargetHotend1(0);
-  setTargetHotend2(0);
-  setTargetHotend3(0);
-  setTargetBed(0);
+  disable_all_heaters();
   fanSpeed = 0;
   lcd_return_to_status();
 }