fix deprecation warnings with new Qt (#435)

* fix deprecation warnings with new Qt

Signed-off-by: William Woodall <william@osrfoundation.org>

* Get QScreen from main window always

Signed-off-by: Jacob Perron <jacob@openrobotics.org>

rviz_common/src/rviz_common/loading_dialog.cpp

@@ -53,7 +53,6 @@ void LoadingDialog::showMessage(const QString & message)
   label_->setText(message);
   QApplication::processEvents();
   QWidget::repaint();
-  QApplication::flush();
 }
 
 }  // namespace rviz_common

rviz_common/src/rviz_common/screenshot_dialog.cpp

@@ -42,8 +42,10 @@
 #include <QMessageBox>  // NOLINT: cpplint is unable to handle the include order here
 // Included so we know that QPushButton inherits QAbstractButton
 #include <QPushButton>  // NOLINT: cpplint is unable to handle the include order here
+#include <QScreen>  // NOLINT: cpplint is unable to handle the include order here
 #include <QTimer>  // NOLINT: cpplint is unable to handle the include order here
 #include <QVBoxLayout>  // NOLINT: cpplint is unable to handle the include order here
+#include <QWindow>  // NOLINT: cpplint is unable to handle the include order here
 
 #include "./scaled_image_widget.hpp"
 
@@ -119,10 +121,11 @@ void ScreenshotDialog::onTimeout()
 
 void ScreenshotDialog::takeScreenshotNow()
 {
+  QScreen * screen = main_window_->windowHandle()->screen();
   if (save_full_window_) {
-    screenshot_ = QPixmap::grabWindow(main_window_->winId());
+    screenshot_ = screen->grabWindow(main_window_->winId());
   } else {
-    screenshot_ = QPixmap::grabWindow(render_window_->winId());
+    screenshot_ = screen->grabWindow(render_window_->winId());
   }
   image_widget_->setImage(screenshot_);
 }