Clean up listener

If the style is unloaded for example by an application dynamically
switching styles, the listener was still around leading us to call
into destroyed objects when the color scheme was changed.
BUG:432660

kstyle/breezetoolsareamanager.cpp

@@ -41,7 +41,7 @@ namespace Breeze {
 
     void ToolsAreaManager::registerApplication(QApplication *application)
     {
-        _listener = new AppListener;
+        _listener = new AppListener(this);
         _listener->manager = this;
         if (application->property(colorProperty).isValid()) {
             auto path = application->property(colorProperty).toString();

kstyle/breezetoolsareamanager.h

@@ -18,6 +18,7 @@ namespace Breeze {
     class AppListener : public QObject
     {
         Q_OBJECT
+        using QObject::QObject;
 
         bool eventFilter(QObject *watched, QEvent *event) override;