owimageviewer: Open local images directly

Using QNetworkAccessManager to 'retrieve' many local files can lead
to temporarily running out of available file descriptors.

Presumably this is because the associated open fd is not closed
until the QNetworkReply is actually deleted (and not just closed).

Orange/widgets/data/owimageviewer.py

@@ -1005,7 +1005,23 @@ def setupScene(self):
                 thumbnail.instance = inst
                 self.thumbnailView.addThumbnail(thumbnail)
 
-                if url.isValid():
+                if url.isValid() and url.isLocalFile():
+                    reader = QImageReader(url.toLocalFile())
+                    image = reader.read()
+                    if image.isNull():
+                        error = reader.errorString()
+                        thumbnail.setToolTip(
+                            thumbnail.toolTip() + "\n" + error)
+                        self._errcount += 1
+                    else:
+                        pixmap = QPixmap.fromImage(image)
+                        thumbnail.setPixmap(pixmap)
+                        self._successcount += 1
+
+                    future = Future()
+                    future.set_result(image)
+                    future._reply = None
+                elif url.isValid():
                     future = self.loader.get(url)
 
                     @future.add_done_callback
@@ -1025,13 +1041,17 @@ def set_pixmap(future, thumb=thumbnail):
 
                         thumb.setPixmap(pixmap)
 
-                        self._updateStatus(future)
-
+                        self._noteCompleted(future)
                 else:
                     future = None
+
                 self.items.append(_ImageItem(i, thumbnail, url, future))
 
-            self.info.setText("Retrieving...\n")
+            if any(it.future is not None and not it.future.done()
+                   for it in self.items):
+                self.info.setText("Retrieving...\n")
+            else:
+                self._updateStatus()
 
     def urlFromValue(self, value):
         variable = value.variable
@@ -1099,7 +1119,8 @@ def commit(self):
         else:
             self.send("Data", None)
 
-    def _updateStatus(self, future):
+    def _noteCompleted(self, future):
+        # Note the completed future's state
         if future.cancelled():
             return
 
@@ -1109,6 +1130,9 @@ def _updateStatus(self, future):
         else:
             self._successcount += 1
 
+        self._updateStatus()
+
+    def _updateStatus(self):
         count = len([item for item in self.items if item.future is not None])
         self.info.setText(
             "Retrieving:\n" +