replace QDesktopWidget() with QGuiApplication.primaryScreen()

the "try, except" could be dropped if we drop Qt4 support.

however, primaryScreen() may not be the right screen.

pyqtgraph/exporters/PrintExporter.py

@@ -45,7 +45,11 @@ class PrintExporter(Exporter):
         #res = printer.resolution()
         sr = self.getSourceRect()
         #res = sr.width() * .4 / (self.params['width'] * 100 / 2.54)
-        res = QtGui.QDesktopWidget().physicalDpiX()
+        try:
+            res = QtGui.QDesktopWidget().physicalDpiX()
+        except AttributeError:
+            # This is available since Qt 5
+            res = QtGui.QGuiApplication.primaryScreen().physicalDotsPerInchX()
         printer.setResolution(res)
         rect = printer.pageRect()
         center = rect.center()

pyqtgraph/exporters/SVGExporter.py

@@ -195,7 +195,11 @@ def _generateItemSvg(item, nodes=None, root=None, options={}):
         buf = QtCore.QBuffer(arr)
         svg = QtSvg.QSvgGenerator()
         svg.setOutputDevice(buf)
-        dpi = QtGui.QDesktopWidget().logicalDpiX()
+        try:
+            dpi = QtGui.QDesktopWidget().logicalDpiX()
+        except AttributeError:
+            # This is available since Qt 5
+            dpi = QtGui.QGuiApplication.primaryScreen().logicalDotsPerInchX()
         svg.setResolution(dpi)
 
         p = QtGui.QPainter()