Set a black background in the video widget

The black background is removed when the playback is starting and set
back again when the video is over. That way the black background does
not change the appearance of the video (and its proportions).

src/GUI/PreviewWidget.cpp

@@ -43,6 +43,12 @@ PreviewWidget::PreviewWidget( GenericRenderer* genericRenderer, QWidget *parent
     m_ui->seekSlider->setSingleStep( 2 );
     m_ui->seekSlider->setFocusPolicy( Qt::NoFocus );
 
+    // Prepare and set the black background
+    m_ui->renderWidget->setAutoFillBackground( true );
+    m_videoPalette = m_ui->renderWidget->palette();
+    m_videoPalette.setColor( QPalette::Window, QColor( Qt::black ) );
+    m_ui->renderWidget->setPalette( m_videoPalette );
+
     setAcceptDrops( false );
 
     connect( m_ui->seekSlider, SIGNAL( sliderPressed() ),       this,   SLOT( seekSliderPressed() ) );
@@ -145,11 +151,17 @@ void    PreviewWidget::videoStopped()
 {
     m_ui->pushButtonPlay->setIcon( QIcon( ":/images/play" ) );
     m_ui->seekSlider->setValue( 0 );
+
+    // Set the black background
+    m_ui->renderWidget->setPalette( m_videoPalette );
 }
 
 void    PreviewWidget::videoPlaying()
 {
     m_ui->pushButtonPlay->setIcon( QIcon( ":/images/pause" ) );
+
+    // Remove the black background
+    m_ui->renderWidget->setPalette( QPalette() );
 }
 
 void    PreviewWidget::endReached()
@@ -158,6 +170,9 @@ void    PreviewWidget::endReached()
 
     m_ui->pushButtonPlay->setIcon( QIcon( ":/images/play" ) );
     m_ui->seekSlider->setValue( 0 );
+
+    // Set the black background
+    m_ui->renderWidget->setPalette( m_videoPalette );
 }
 
 void        PreviewWidget::on_pushButtonNextFrame_clicked()

src/GUI/PreviewWidget.h

@@ -47,6 +47,7 @@ private:
     GenericRenderer*        m_renderer;
     bool                    m_endReached;
     bool                    m_previewStopped;
+    QPalette                m_videoPalette;
 
 protected:
     virtual void    changeEvent( QEvent *e );