Qt: fix couple memory leaks

ui/drivers/qt/ui_qt_window.cpp

@@ -710,6 +710,8 @@ MainWindow::~MainWindow()
       delete m_thumbnailPixmap2;
    if (m_thumbnailPixmap3)
       delete m_thumbnailPixmap3;
+   if (m_proxyFileModel)
+      delete m_proxyFileModel;
 }
 
 void MainWindow::startTimer() {

ui/drivers/ui_qt.cpp

@@ -170,6 +170,8 @@ void ThumbnailLabel::paintEvent(QPaintEvent *event)
 
    if (!m_pixmap || m_pixmap->isNull())
    {
+      if (m_pixmap)
+         delete m_pixmap;
       m_pixmap = new QPixmap(sizeHint());
       m_pixmap->fill(QColor(0, 0, 0, 0));
    }