[partition] Replace useless qobject_cast

- We know the type of the widget, but were declaring it as
   a QWidget. Just forward-declare the class and be done with it.

src/modules/partition/gui/PartitionViewStep.cpp

@@ -75,7 +75,7 @@ PartitionViewStep::PartitionViewStep( QObject* parent )
 
     m_waitingWidget = new WaitingWidget( QString() );
     m_widget->addWidget( m_waitingWidget );
-    CALAMARES_RETRANSLATE( qobject_cast< WaitingWidget* >( m_waitingWidget )->setText( tr( "Gathering system information..." ) ); )
+    CALAMARES_RETRANSLATE( m_waitingWidget->setText( tr( "Gathering system information..." ) ); )
 
     m_core = new PartitionCoreModule( this ); // Unusable before init is complete!
     // We're not done loading, but we need the configuration map first.

src/modules/partition/gui/PartitionViewStep.h

@@ -35,6 +35,7 @@ class ChoicePage;
 class PartitionPage;
 class PartitionCoreModule;
 class QStackedWidget;
+class WaitingWidget;
 
 template<typename T> class QFutureWatcher;
 
@@ -82,7 +83,7 @@ private:
     ChoicePage*       m_choicePage;
     PartitionPage*    m_manualPartitionPage;
 
-    QWidget*          m_waitingWidget;
+    WaitingWidget* m_waitingWidget;
     QFutureWatcher<void>* m_future;
 
     QSet< PartitionActions::Choices::SwapChoice > m_swapChoices;