Extensions: fix Qt freeze when reloading scripts

Description of the bug:

 This freeze happened only on Windows (tested on XP & Wine), but not on
 Linux. If I only moved up the "killInstance()" line, then the freeze
 would happen only on Linux but not on Windows.
 In these cases the QDialog's destructor never returned.

modules/gui/qt4/dialogs/extensions.cpp

@@ -207,11 +207,6 @@ ExtensionDialog::ExtensionDialog( intf_thread_t *_p_intf,
 ExtensionDialog::~ExtensionDialog()
 {
     msg_Dbg( p_intf, "Deleting extension dialog '%s'", qtu(windowTitle()) );
-    /* Delete all widgets */
-    extension_widget_t *p_widget;
-    p_dialog->b_kill = true;
-    p_dialog->p_sys_intf = NULL;
-    vlc_cond_signal( &p_dialog->cond );
 }
 
 QWidget* ExtensionDialog::CreateWidget( extension_widget_t *p_widget )

modules/gui/qt4/extensions_manager.cpp

@@ -118,11 +118,10 @@ void ExtensionsManager::unloadExtensions()
     if( !p_extensions_manager )
         return;
     b_unloading = true;
+    ExtensionsDialogProvider::killInstance();
     module_unneed( p_extensions_manager, p_extensions_manager->p_module );
     vlc_object_release( p_extensions_manager );
     p_extensions_manager = NULL;
-    emit extensionsUpdated();
-    ExtensionsDialogProvider::killInstance();
 }
 
 void ExtensionsManager::reloadExtensions()