Fix several compiler warnings

* fix -Winconsistent-missing-override
* fix -Wunused-but-set-variable
* fix -Wunused-parameter
* fix -Wunused-lambda-capture
* fix -Wunused-private-field
* fix -Wpessimizing-move

src/Gui/Application.cpp

@@ -803,8 +803,7 @@ void Application::createStandardOperations()
 void Application::slotNewDocument(const App::Document& Doc, bool isMainDoc)
 {
 #ifdef FC_DEBUG
-    std::map<const App::Document*, Gui::Document*>::const_iterator it = d->documents.find(&Doc);
-    assert(it==d->documents.end());
+    assert(d->documents.find(&Doc) == d->documents.end());
 #endif
     auto pDoc = new Gui::Document(const_cast<App::Document*>(&Doc),this);
     d->documents[&Doc] = pDoc;

src/Gui/WorkbenchSelector.cpp

@@ -238,7 +238,7 @@ void WorkbenchTabWidget::buildPrefMenu()
     menu->addSeparator();
 
     QAction* preferencesAction = menu->addAction(tr("Preferences"));
-    connect(preferencesAction, &QAction::triggered, this, [this]() {
+    connect(preferencesAction, &QAction::triggered, this, []() {
         Gui::Dialog::DlgPreferencesImp cDlg(getMainWindow());
         cDlg.activateGroupPage(QString::fromUtf8("Workbenches"), 0);
         cDlg.exec();