Add status_printf to print messages to the lcd status line.

Also add a hotend indicator to the heating message displayed on the lcd status line.

Marlin/Marlin_main.cpp

       }
     #endif
 
-    if (code_value_temp_abs() > thermalManager.degHotend(target_extruder)) LCD_MESSAGEPGM(MSG_HEATING);
+    if (code_value_temp_abs() > thermalManager.degHotend(target_extruder)) status_printf(0, "E%i %s", target_extruder + 1, MSG_HEATING);
   }
 
   #if ENABLED(AUTOTEMP)
       else print_job_timer.start();
     #endif
 
-    if (thermalManager.isHeatingHotend(target_extruder)) LCD_MESSAGEPGM(MSG_HEATING);
+    if (thermalManager.isHeatingHotend(target_extruder)) status_printf(0, "E%i %s", target_extruder + 1, MSG_HEATING);
   }
 
   #if ENABLED(AUTOTEMP)

Marlin/ultralcd.cpp

   lcd_finishstatus(level > 0);
 }
 
+void status_printf(uint8_t level, const char *status, ...) {
+  if (level < lcd_status_message_level) return;
+  lcd_status_message_level = level;
+  va_list args;
+  va_start(args, status);
+  vsnprintf(lcd_status_message, 3 * (LCD_WIDTH), status, args);
+  va_end(args);
+  lcd_finishstatus(level > 0);
+}
+
 void lcd_setalertstatuspgm(const char* const message) {
   lcd_setstatuspgm(message, 1);
   #if ENABLED(ULTIPANEL)

Marlin/ultralcd.h

   bool lcd_hasstatus();
   void lcd_setstatus(const char* message, const bool persist=false);
   void lcd_setstatuspgm(const char* message, const uint8_t level=0);
+  void status_printf(uint8_t level, const char *Status, ...);
   void lcd_setalertstatuspgm(const char* message);
   void lcd_reset_alert_level();
   void lcd_kill_screen();
   inline bool lcd_hasstatus() { return false; }
   inline void lcd_setstatus(const char* const message, const bool persist=false) { UNUSED(message); UNUSED(persist); }
   inline void lcd_setstatuspgm(const char* const message, const uint8_t level=0) { UNUSED(message); UNUSED(level); }
+  inline void status_printf(uint8_t level, const char *status, ...) { UNUSED(level); UNUSED(status); }
   inline void lcd_buttons_update() {}
   inline void lcd_reset_alert_level() {}
   inline bool lcd_detected() { return true; }