Implementation of Link

This patch includes the actual implementation of Link, which is
implemented as an extension named LinkBaseExtension in App namespace,
and a full view provider ViewProviderLink in Gui. The reason of not
using ViewProviderExtension is because it need full control when
display, not just extending existing functionalities.

Please see [here](https://git.io/fjPue) for more details of the
implementation.

This patch also includes a set of link manipulation commands, and a
task panel for overriding geometry element colors.

src/Gui/ViewProviderLinkPyImp.cpp

@@ -0,0 +1,83 @@
+/****************************************************************************
+ *   Copyright (c) 2017 Zheng, Lei (realthunder) <realthunder.dev@gmail.com>*
+ *                                                                          *
+ *   This file is part of the FreeCAD CAx development system.               *
+ *                                                                          *
+ *   This library is free software; you can redistribute it and/or          *
+ *   modify it under the terms of the GNU Library General Public            *
+ *   License as published by the Free Software Foundation; either           *
+ *   version 2 of the License, or (at your option) any later version.       *
+ *                                                                          *
+ *   This library  is distributed in the hope that it will be useful,       *
+ *   but WITHOUT ANY WARRANTY; without even the implied warranty of         *
+ *   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the          *
+ *   GNU Library General Public License for more details.                   *
+ *                                                                          *
+ *   You should have received a copy of the GNU Library General Public      *
+ *   License along with this library; see the file COPYING.LIB. If not,     *
+ *   write to the Free Software Foundation, Inc., 59 Temple Place,          *
+ *   Suite 330, Boston, MA  02111-1307, USA                                 *
+ *                                                                          *
+ ****************************************************************************/
+
+#include "PreCompiled.h"
+
+#ifndef _PreComp_
+# include <sstream>
+#endif
+
+#include "Gui/ViewProviderLink.h"
+#include <App/DocumentObject.h>
+#include <Base/PlacementPy.h>
+// inclusion of the generated files (generated out of ViewProviderLinkPy.xml)
+#include "ViewProviderLinkPy.h"
+#include "ViewProviderLinkPy.cpp"
+
+using namespace Gui;
+
+// returns a string which represents the object e.g. when printed in python
+std::string ViewProviderLinkPy::representation(void) const
+{
+    std::stringstream str;
+    str << "<ViewProviderLink at " << getViewProviderLinkPtr() << ">";
+
+    return str.str();
+}
+
+Py::Object ViewProviderLinkPy::getDraggingPlacement() const {
+    return Py::Object(new Base::PlacementPy(new Base::Placement(
+                    getViewProviderLinkPtr()->currentDraggingPlacement())));
+}
+
+void ViewProviderLinkPy::setDraggingPlacement(Py::Object arg) {
+    if(!PyObject_TypeCheck(arg.ptr(),&Base::PlacementPy::Type))
+        throw Py::TypeError("expects a placement");
+    getViewProviderLinkPtr()->updateDraggingPlacement(
+            *static_cast<Base::PlacementPy*>(arg.ptr())->getPlacementPtr());
+}
+
+Py::Boolean ViewProviderLinkPy::getUseCenterballDragger() const {
+    return Py::Boolean(getViewProviderLinkPtr()->isUsingCenterballDragger());
+}
+
+void ViewProviderLinkPy::setUseCenterballDragger(Py::Boolean arg) {
+    try {
+        getViewProviderLinkPtr()->enableCenterballDragger(arg);
+    }catch(const Base::Exception &e){
+        throw Py::Exception(Base::BaseExceptionFreeCADError,e.what());
+    }
+}
+
+Py::Object ViewProviderLinkPy::getLinkView() const {
+    return Py::Object(getViewProviderLinkPtr()->getPyLinkView(),true);
+}
+
+PyObject *ViewProviderLinkPy::getCustomAttributes(const char* /*attr*/) const
+{
+    return 0;
+}
+
+int ViewProviderLinkPy::setCustomAttributes(const char* /*attr*/, PyObject* /*obj*/)
+{
+    return 0; 
+}