PropertyExpressionEngine: convert to link type property

PropertyExpressionEngine is changed to derived from a new class
PropertyExpressionContainer, which is in turn derives from
PropertyXLinkContainer. This makes PropertyExpressionEngine a link type
property that is capable of external linking. It now uses the unified
link property APIs for dependency management and tracking of object
life time, re-labeling, etc.

ObjectIdentifier is modified to support sub-object reference, but is
not exposed to end-user, because expression syntax is kept mostly
unchanged, which will be submitted in future PR. There is, however,
one small change in expression syntax (ExpressionParser.y) to introduce
local property reference to avoid ambiguity mentioned in
FreeCAD/FreeCAD#1619

Modified Expression/ExpressionModifier interface to support various link
property API for link modification.

src/App/DocumentObjectPy.xml

@@ -176,6 +176,19 @@ non-object sub-element name if any.
                 </UserDocu>
             </Documentation>
         </Methode>
+        <Methode Name="resolveSubElement" Const="true">
+            <Documentation>
+                <UserDocu>
+resolveSubElement(subname,append,type) -- resolve both new and old style sub element
+
+subname: subname reference contianing object hierarchy
+append: Whether to append object hierarchy prefix inside subname to returned element name
+type: 0: normal, 1: for import, 2: for export
+
+Return tuple(obj,newElementName,oldElementName)
+                </UserDocu>
+            </Documentation>
+        </Methode>
         <Methode Name="adjustRelativeLinks">
             <Documentation>
                 <UserDocu>adjustRelativeLinks(parent,recursive=True) -- auto correct potential cyclic dependencies</UserDocu>