Handle Long-Press in the main loop

The longpress function is currently called within the temperature ISR,
which is bogus. Calling the longpress function at the wrong moment
can corrupt the menu buffers.

Move the call to the main loop by changing the logic slightly:

- still sample the lcd buttons inside the temperature ISR, which keeps
  scrollong/pressing responsive, but...
- set a flag to indicate that longpress has been triggered instead of
  calling the function directly
- call the function on the next manage_inactivity run

Combined with #3180 this removes _most_ unsafe operations out of the ISR
which can happen during a normal run (max/mintemp warnings are still an
exception).

Firmware/Marlin_main.cpp

@@ -9901,6 +9901,17 @@ if(0)
   #endif
   check_axes_activity();
   mmu_loop();
+
+  // handle longpress
+  if(lcd_longpress_trigger)
+  {
+      // long press is not possible in modal mode, wait until ready
+      if (lcd_longpress_func && lcd_update_enabled)
+      {
+          lcd_longpress_func();
+          lcd_longpress_trigger = 0;
+      }
+  }
 }
 
 void kill(const char *full_screen_message, unsigned char id)