Gui: modernize C++11

* use nullptr

src/Gui/TextEdit.cpp

@@ -40,7 +40,7 @@ using namespace Gui;
  *  Constructs a TextEdit which is a child of 'parent'.
  */
 TextEdit::TextEdit(QWidget* parent)
-    : QPlainTextEdit(parent), cursorPosition(0), listBox(0)
+    : QPlainTextEdit(parent), cursorPosition(0), listBox(nullptr)
 {
     //Note: Set the correct context to this shortcut as we may use several instances of this
     //class at a time
@@ -219,7 +219,7 @@ struct TextEditorP
  *  syntax highlighting for the Python language.
  */
 TextEditor::TextEditor(QWidget* parent)
-  : TextEdit(parent), WindowParameter("Editor"), highlighter(0)
+  : TextEdit(parent), WindowParameter("Editor"), highlighter(nullptr)
 {
     d = new TextEditorP();
     lineNumberArea = new LineMarker(this);