Base: Rename Base::freecad_dynamic_cast into freecad_cast

This is to make it shorter and easier to use. QT does the same thing
with their qobject_cast.

src/App/GroupExtension.cpp

@@ -414,7 +414,7 @@ bool GroupExtension::extensionGetSubObject(DocumentObject*& ret,
 {
     const char* dot;
     if (!subname || *subname == 0) {
-        auto obj = Base::freecad_dynamic_cast<const DocumentObject>(getExtendedContainer());
+        auto obj = freecad_cast<const DocumentObject>(getExtendedContainer());
         ret = const_cast<DocumentObject*>(obj);
         return true;
     }