PropertyLinks: refactor property link API

* Create new class PropertyLinkBase as the common parent class for all
  type of links. See the class document for details of its API.

* Added new link scope 'Hidden' that is ignored during normal object
  dependency calculation, but still keep tracks of object remove,
  relabel, etc.

* There is a new concept called 'Shadow subname' introduced in this
  patch, which will only be meaningful in future topological naming
  feature. See [here](https://git.io/fjXKR) for more details.

* DocumentObject added a new API adjustRelativeLink() and a helper
  resolveRelativeLink() function.

src/App/DocumentObjectPy.xml

@@ -176,6 +176,11 @@ non-object sub-element name if any.
                 </UserDocu>
             </Documentation>
         </Methode>
+        <Methode Name="adjustRelativeLinks">
+            <Documentation>
+                <UserDocu>adjustRelativeLinks(parent,recursive=True) -- auto correct potential cyclic dependencies</UserDocu>
+            </Documentation>
+        </Methode>
     <Attribute Name="OutList" ReadOnly="true">
       <Documentation>
         <UserDocu>A list of all objects this object links to.</UserDocu>