Improve ExtUI, fix compiler errors, warnings (#14441)

Marlin/src/gcode/queue.h

@@ -84,6 +84,11 @@ public:
    */
   static void enqueue_now_P(PGM_P const cmd);
 
+  /**
+   * Check whether there are any commands yet to be executed
+   */
+  static bool has_commands_queued();
+
   /**
    * Get the next command in the queue, optionally log it to SD, then dispatch it
    */