[kstyle] Implement application unpolish to delete ShadowHelper

Summary:
Another change mostly for KWin (Wayland compositor). KWin destroys it's
internal Wayland connection prior to the QStyle getting destroyed. As
the ShadowHelper initialized Wayland objects those would be destroyed
after the connection is destroyed. With latest Wayland library this
would cause a crash.

Thus unpolish is implemented in the style and deletes the ShadowHelper.
KWin can on tear down invoke the unpolish and thus make sure it doesn't
crash.

CCBUG: 372001

Reviewers: #plasma, broulik, hpereiradacosta

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D3240

kstyle/breezestyle.cpp

@@ -197,9 +197,24 @@ namespace Breeze
 
     //______________________________________________________________
     Style::~Style( void )
+    {
+        cleanup();
+    }
+
+    //______________________________________________________________
+    void Style::unpolish( QApplication* )
+    {
+        // NOTE: unpolish is not called in general on tear-down of Application
+        // only when a new QStyle is loaded by the QApplication unpolish is invoked
+        cleanup();
+    }
+
+    void Style::cleanup( void )
     {
         delete _shadowHelper;
+        _shadowHelper = nullptr;
         delete _helper;
+        _helper = nullptr;
     }
 
     //______________________________________________________________

kstyle/breezestyle.h

@@ -92,7 +92,6 @@ namespace Breeze
 
         //* needed to avoid warnings at compilation time
         using  ParentStyleClass::polish;
-        using  ParentStyleClass::unpolish;
 
         //* widget polishing
         virtual void polish( QWidget* );
@@ -100,6 +99,9 @@ namespace Breeze
         //* widget unpolishing
         virtual void unpolish( QWidget* );
 
+        //* application unpolishing
+        void unpolish( QApplication* ) override;
+
         //* polish scrollarea
         void polishScrollArea( QAbstractScrollArea* );
 
@@ -471,6 +473,9 @@ namespace Breeze
 
         private:
 
+        //* shared cleanup for unpolish and dtor
+        void cleanup();
+
         //*@name scrollbar button types (for addLine and subLine )
         //@{
         ScrollBarButtonType _addLineButtons;