SvxFontNameBox_Impl was not disposed

This is visible after toolbar customization, when the old
font box stays visible. Turns out that currently the toolbox
controller is supposed to own the item window, not the toolbox.

Regression of:

commit 970a66f8
Author: Maxim Monastirsky <momonasmon@gmail.com>
Date:   Sun Oct 16 01:27:10 2016 +0300

    Use UNO registration for font name toolbox control

Change-Id: I29a1f06d97c9a981507ff62889e2d7fc6ac9d688
Reviewed-on: https://gerrit.libreoffice.org/32943Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Maxim Monastirsky <momonasmon@gmail.com>

svx/source/tbxctrls/tbcontrl.cxx

@@ -2683,10 +2683,16 @@ public:
     // XToolbarController
     virtual css::uno::Reference< css::awt::XWindow > SAL_CALL createItemWindow( const css::uno::Reference< css::awt::XWindow >& rParent ) throw ( css::uno::RuntimeException, std::exception ) override;
 
+    // XComponent
+    virtual void SAL_CALL dispose() throw ( css::uno::RuntimeException, std::exception ) override;
+
     // XServiceInfo
     virtual OUString SAL_CALL getImplementationName() throw ( css::uno::RuntimeException, std::exception ) override;
     virtual sal_Bool SAL_CALL supportsService( const OUString& rServiceName ) throw ( css::uno::RuntimeException, std::exception ) override;
     virtual css::uno::Sequence< OUString > SAL_CALL getSupportedServiceNames() throw ( css::uno::RuntimeException, std::exception ) override;
+
+private:
+    VclPtr<SvxFontNameBox_Impl> m_pBox;
 };
 
 SvxFontNameToolBoxControl::SvxFontNameToolBoxControl()
@@ -2702,25 +2708,21 @@ void SvxFontNameToolBoxControl::statusChanged( const css::frame::FeatureStateEve
     if ( !getToolboxId( nId, &pToolBox ) )
         return;
 
-    SvxFontNameBox_Impl* pBox = static_cast<SvxFontNameBox_Impl*>(pToolBox->GetItemWindow( nId ));
-
-    DBG_ASSERT( pBox, "Control not found!" );
-
     if ( !rEvent.IsEnabled )
     {
-        pBox->Disable();
-        pBox->Update( nullptr );
+        m_pBox->Disable();
+        m_pBox->Update( nullptr );
     }
     else
     {
-        pBox->Enable();
+        m_pBox->Enable();
 
         css::awt::FontDescriptor aFontDesc;
         if ( rEvent.State >>= aFontDesc )
-            pBox->Update( &aFontDesc );
+            m_pBox->Update( &aFontDesc );
         else
-            pBox->SetText( "" );
-        pBox->SaveValue();
+            m_pBox->SetText( "" );
+        m_pBox->SaveValue();
     }
 
     pToolBox->EnableItem( nId, rEvent.IsEnabled );
@@ -2730,10 +2732,17 @@ css::uno::Reference< css::awt::XWindow > SvxFontNameToolBoxControl::createItemWi
     throw ( css::uno::RuntimeException, std::exception )
 {
     SolarMutexGuard aGuard;
-    VclPtrInstance<SvxFontNameBox_Impl> pBox( VCLUnoHelper::GetWindow( rParent ),
-                                              Reference< XDispatchProvider >( m_xFrame->getController(), UNO_QUERY ),
-                                              m_xFrame,0);
-    return VCLUnoHelper::GetInterface( pBox );
+    m_pBox = VclPtr<SvxFontNameBox_Impl>::Create( VCLUnoHelper::GetWindow( rParent ),
+                                                  Reference< XDispatchProvider >( m_xFrame->getController(), UNO_QUERY ),
+                                                  m_xFrame, 0);
+    return VCLUnoHelper::GetInterface( m_pBox );
+}
+
+void SvxFontNameToolBoxControl::dispose()
+    throw ( css::uno::RuntimeException, std::exception )
+{
+    m_pBox.disposeAndClear();
+    ToolboxController::dispose();
 }
 
 OUString SvxFontNameToolBoxControl::getImplementationName()