Qt: Fix some modality

Close #2763

modules/gui/qt4/dialogs/help.cpp

@@ -85,6 +85,7 @@ AboutDialog::AboutDialog( intf_thread_t *_p_intf)
     setWindowRole( "vlc-about" );
     resize( 600, 500 );
     setMinimumSize( 600, 500 );
+    setWindowModality( Qt::WindowModal );
 
     QGridLayout *layout = new QGridLayout( this );
     QTabWidget *tab = new QTabWidget( this );

modules/gui/qt4/dialogs/preferences.cpp

@@ -45,6 +45,7 @@ PrefsDialog::PrefsDialog( QWidget *parent, intf_thread_t *_p_intf )
     QGridLayout *main_layout = new QGridLayout( this );
     setWindowTitle( qtr( "Preferences" ) );
     setWindowRole( "vlc-preferences" );
+    setWindowModality( Qt::WindowModal );
 
     /* Whether we want it or not, we need to destroy on close to get
        consistency when reset */