Qt: correctly clear the info panel cover Art

Manual cherry-pick of 2ea0e546475dac1dd9032a06937966ed3d115ca9

modules/gui/qt4/components/info_panels.cpp

@@ -327,6 +327,7 @@ void MetaPanel::clear()
     nowplaying_text->clear();
     publisher_text->clear();
     encodedby_text->clear();
+    art_cover->clear();
 
     setEditMode( false );
     emit uriSet( "" );

modules/gui/qt4/components/interface_widgets.cpp

@@ -556,6 +556,11 @@ void CoverArtLabel::askForUpdate()
     THEMIM->getIM()->requestArtUpdate();
 }
 
+void CoverArtLabel::clear()
+{
+    showArtUpdate( "" );
+}
+
 TimeLabel::TimeLabel( intf_thread_t *_p_intf, TimeLabel::Display _displayType  )
     : QLabel(), p_intf( _p_intf ), bufTimer( new QTimer(this) ),
       buffering( false ), showBuffering(false), bufVal( -1 ), displayType( _displayType )

modules/gui/qt4/components/interface_widgets.hpp

@@ -238,6 +238,7 @@ public slots:
         requestUpdate();
     }
     void showArtUpdate( const QString& );
+    void clear();
 
 private slots:
     void askForUpdate();