Automatically reset stepper timeout in manage_inactivity

Any code that adds moves to the planner can skip resetting the stepper timeout. We can let `idle` / `manage_inactivity` reset the timer whenever it detects any moves in the planner.

Marlin/planner.h

@@ -508,7 +508,7 @@ class Planner {
     /**
      * Does the buffer have any blocks queued?
      */
-    static bool blocks_queued() { return (block_buffer_head != block_buffer_tail); }
+    static inline bool blocks_queued() { return (block_buffer_head != block_buffer_tail); }
 
     /**
      * "Discard" the block and "release" the memory.