Add a helper to check whether the transport plays

src/sfizz/BeatClock.h

@@ -110,6 +110,10 @@ public:
      * @brief Set whether the clock is ticking or stopped.
      */
     void setPlaying(unsigned delay, bool playing);
+    /**
+     * Check whether the clock is currently ticking.
+     */
+    bool isPlaying() const noexcept { return isPlaying_; }
     /**
      * @brief Get the beat number for each frame of the current cycle.
      *