[libcalamaresui] Instantiate QML at load time

- By instantiating only on activation, an ugly "white" gap
   appears where there is no widget at all. So instantiate
   earlier so that the widget already exists and is painting
   by the time the slideshow part is visible.
 - This makes the net effect of this branch so far zero:
   the slideshow is still loaded and started when Calamares starts.

src/libcalamaresui/ExecutionViewStep.cpp

@@ -143,12 +143,6 @@ ExecutionViewStep::loadQml()
                                             QQmlComponent::CompilationMode::Asynchronous
                                           );
     }
-}
-
-void
-ExecutionViewStep::onActivate()
-{
-    loadQml();
     if ( m_qmlComponent )
     {
         QObject* o = m_qmlComponent->create();
@@ -164,6 +158,12 @@ ExecutionViewStep::onActivate()
             m_qmlShow->setContent( QUrl::fromLocalFile( Calamares::Branding::instance()->slideshowPath() ), m_qmlComponent, m_qmlObject );
         }
     }
+}
+
+void
+ExecutionViewStep::onActivate()
+{
+    loadQml();
 
     JobQueue* queue = JobQueue::instance();
     foreach ( const QString& instanceKey, m_jobInstanceKeys )