Make DeocrationPlugin a KWin singleton

Like so many other classes there is exactly one instance hold by
Workspace.

plugins.cpp

@@ -30,7 +30,9 @@ along with this program.  If not, see <http://www.gnu.org/licenses/>.
 namespace KWin
 {
 
-DecorationPlugin::DecorationPlugin()
+KWIN_SINGLETON_FACTORY(DecorationPlugin)
+
+DecorationPlugin::DecorationPlugin(QObject *)
     : KDecorationPlugins(KGlobal::config())
     , m_noDecoration(false)
 {
@@ -45,6 +47,11 @@ DecorationPlugin::DecorationPlugin()
 #endif
 }
 
+DecorationPlugin::~DecorationPlugin()
+{
+    s_self = NULL;
+}
+
 void DecorationPlugin::error(const QString &error_msg)
 {
     qWarning("%s", QString(i18n("KWin: ") + error_msg).toLocal8Bit().data());

plugins.h

@@ -23,6 +23,7 @@ along with this program.  If not, see <http://www.gnu.org/licenses/>.
 #define KWIN_PLUGINS_H
 
 #include <kdecoration_plugins_p.h>
+#include <kwinglobals.h>
 
 namespace KWin
 {
@@ -31,7 +32,7 @@ class DecorationPlugin
     : public KDecorationPlugins
 {
 public:
-    DecorationPlugin();
+    virtual ~DecorationPlugin();
     virtual bool provides(Requirement);
     /**
      * @returns @c true if there is no decoration plugin.
@@ -42,6 +43,7 @@ protected:
 private:
     void setNoDecoration(bool noDecoration);
     bool m_noDecoration;
+    KWIN_SINGLETON(DecorationPlugin)
 };
 
 } // namespace

workspace.cpp

@@ -159,7 +159,7 @@ Workspace::Workspace(bool restore)
     reparseConfigFuture.waitForFinished();
     options->loadConfig();
     options->loadCompositingConfig(false);
-    mgr = new DecorationPlugin;
+    mgr = DecorationPlugin::create(this);
     default_colormap = DefaultColormap(display(), screen_number);
     installed_colormap = default_colormap;
 
@@ -534,7 +534,7 @@ Workspace::~Workspace()
 
     delete rootInfo;
     delete supportWindow;
-    delete mgr;
+    delete DecorationManager::self();
     delete startup;
     delete Placement::self();
     delete client_keys_dialog;