App/Gui: improve expression binding of PropertyEnumeration

The enumeration items are exposed through sub path '.Enum'. When
'ShowAll' is enabled in property view, this sub path is exposed as a
sub property item named 'Enum', and can be either manually edited or
bound with an expression.

src/App/Property.cpp

@@ -71,6 +71,8 @@ std::string Property::getFullName() const {
     if(myName) {
         if(father)
             name = father->getFullName() + ".";
+        else
+            name = "?.";
         name += myName;
     }else
         return "?";