VST Host: Ensure parent window is showing before attempting to open an editor

This fixes an issue with NI Raum and Choral where the editors would open
in a new window, rather than in the same window as the VST
AudioProcessorEditor.

modules/juce_audio_processors/format_types/juce_VSTPluginFormat.cpp

@@ -2855,7 +2855,7 @@ public:
     {
         if (cocoaWrapper != nullptr)
         {
-            if (isVisible())
+            if (isShowing())
                 openPluginWindow ((NSView*) cocoaWrapper->getView());
             else
                 closePluginWindow();
@@ -2873,6 +2873,8 @@ public:
                 setSize (w, h);
         }
     }
+
+    void parentHierarchyChanged() override { visibilityChanged(); }
    #else
     void paint (Graphics& g) override
     {