Gui: modernize C++: use range-based for loop

src/Gui/TaskView/TaskDialog.cpp

@@ -48,9 +48,9 @@ TaskDialog::TaskDialog()
 
 TaskDialog::~TaskDialog()
 {
-    for (std::vector<QWidget*>::iterator it=Content.begin();it!=Content.end();++it) {
-        delete *it;
-        *it = 0;
+    for (auto it : Content) {
+        delete it;
+        it = nullptr;
     }
 }