AU Client: Use ScopedJuceInitialiser_GUI to ensure the MessageManager exists

This makes the AU Client compatible with other classes also using the
ScopedJuceInitialiser_GUI, specifically the ARADocumentController.

modules/juce_audio_plugin_client/juce_audio_plugin_client_AU_1.mm

@@ -89,18 +89,14 @@ template <> struct ContainerDeletePolicy<const __CFString>   { static void destr
 // make sure the audio processor is initialized before the AUBase class
 struct AudioProcessorHolder
 {
-    AudioProcessorHolder (bool initialiseGUI)
+    AudioProcessorHolder()
     {
-        if (initialiseGUI)
-            initialiseJuce_GUI();
-
-        juceFilter = createPluginFilterOfType (AudioProcessor::wrapperType_AudioUnit);
-
         // audio units do not have a notion of enabled or un-enabled buses
         juceFilter->enableAllBuses();
     }
 
-    std::unique_ptr<AudioProcessor> juceFilter;
+    ScopedJuceInitialiser_GUI scopedInitialiser;
+    std::unique_ptr<AudioProcessor> juceFilter { createPluginFilterOfType (AudioProcessor::wrapperType_AudioUnit) };
 };
 
 //==============================================================================
@@ -111,8 +107,7 @@ class JuceAU final : public AudioProcessorHolder,
 {
 public:
     JuceAU (AudioUnit component)
-        : AudioProcessorHolder (activePlugins.size() + activeUIs.size() == 0),
-          MusicDeviceBase (component,
+        : MusicDeviceBase (component,
                            (UInt32) AudioUnitHelpers::getBusCountForWrapper (*juceFilter, true),
                            (UInt32) AudioUnitHelpers::getBusCountForWrapper (*juceFilter, false))
     {
@@ -173,9 +168,6 @@ public:
 
         jassert (activePlugins.contains (this));
         activePlugins.removeFirstMatchingValue (this);
-
-        if (activePlugins.size() + activeUIs.size() == 0)
-            shutdownJuce_GUI();
     }
 
     //==============================================================================
@@ -1723,6 +1715,7 @@ public:
         }
 
     private:
+        ScopedJuceInitialiser_GUI scopedInitialiser;
         Rectangle<int> lastBounds;
 
         JUCE_DECLARE_NON_COPYABLE (EditorCompHolder)
@@ -1823,8 +1816,6 @@ public:
                 // there's some kind of component currently modal, but the host
                 // is trying to delete our plugin..
                 jassert (Component::getCurrentlyModalComponent() == nullptr);
-
-                shutdownJuce_GUI();
             }
         }
     };

modules/juce_events/messages/juce_Initialisation.h

@@ -74,6 +74,9 @@ public:
 
     /** The destructor simply calls shutdownJuce_GUI(). */
     ~ScopedJuceInitialiser_GUI();
+
+    JUCE_DECLARE_NON_COPYABLE (ScopedJuceInitialiser_GUI)
+    JUCE_DECLARE_NON_MOVEABLE (ScopedJuceInitialiser_GUI)
 };