Fix clang compiler warnings:

+ fix -Winconsistent-missing-override
+ fix -Wunused-variable
+ fix -Wbraced-scalar-init
+ fix -Wparentheses (View3DInventorViewer::checkGroupOnTop: operator '?:' has lower precedence than '+'; '+' will be evaluated first)
+ fix -Wundefined-bool-conversion (MainWindow::updateActions)
+ suppress -Woverloaded-virtual but fix later

src/Gui/ExpressionCompleter.cpp

@@ -259,7 +259,6 @@ public:
     }
 
     int rowCount(const QModelIndex & parent = QModelIndex()) const {
-        const auto &docs = App::GetApplication().getDocuments();
         Info info;
         int row = 0;
         if(!parent.isValid()) {