Gui: modernize C++11

* use nullptr

src/Gui/View.cpp

@@ -69,7 +69,7 @@ void BaseView::onClose(void)
             _pcDocument->detachView(this);
     }
 
-    _pcDocument = 0;
+    _pcDocument = nullptr;
     bIsDetached = true;
 }
 
@@ -95,5 +95,5 @@ void BaseView::setDocument(Gui::Document* pcDocument)
 /// returns the document the view is attached to
 App::Document* BaseView::getAppDocument() const
 {
-    return _pcDocument ? _pcDocument->getDocument() : 0;
+    return _pcDocument ? _pcDocument->getDocument() : nullptr;
 }