make sample rate changeable, probably

Source/FluidSynthModel.h

@@ -59,6 +59,8 @@ public:
     */
     void removeListener (Listener* listener);
 
+    void setSampleRate(float sampleRate);
+
     const String& getCurrentSoundFontAbsPath();
 
 private:
@@ -70,6 +72,8 @@ private:
 
     String currentSoundFontAbsPath;
 
+    float currentSampleRate;
+
     const fluid_preset_t getFirstPreset();
     void selectFirstPreset();
     unique_ptr<BankAndPreset> getFirstBankAndPreset();