fix macOS build (following Projucer changes made in Windows, which removed /Applications/JUCE/modules from its headers). move JUCE headers under source control, so that Windows and macOS can both build against same version of JUCE. remove AUv3 target (I think it's an iOS thing, so it will never work with this macOS fluidsynth dylib).

modules/juce_core/native/java/JuceFirebaseInstanceIdService.java

@@ -0,0 +1,16 @@
+package com.juce;
+
+import com.google.firebase.iid.*;
+
+public final class JuceFirebaseInstanceIdService extends FirebaseInstanceIdService
+{
+    private native void firebaseInstanceIdTokenRefreshed (String token);
+
+    @Override
+    public void onTokenRefresh()
+    {
+        String token = FirebaseInstanceId.getInstance().getToken();
+
+        firebaseInstanceIdTokenRefreshed (token);
+    }
+}