App: extend Expression syntax

* Support sub-object reference syntax using the following syntax,
    Part.<<Box.>>.Placement
  or, with sub-object label referencing
    Part.<<$Cube.>>.Placement

* Extend indexing support, including range based indexing, e.g.
    A1[B2+1][C3][D4:-1]

* Add new constants, None, True, true, False, false.

src/Mod/Spreadsheet/App/Sheet.cpp

@@ -692,7 +692,11 @@ void Sheet::updateProperty(CellAddress key)
         auto number = freecad_dynamic_cast<NumberExpression>(output.get());
         if(number) {
             long l;
-            if (!number->getUnit().isEmpty())
+            auto constant = freecad_dynamic_cast<ConstantExpression>(output.get());
+            if(constant && !constant->isNumber()) {
+                Base::PyGILStateLocker lock;
+                setObjectProperty(key, constant->getPyValue());
+            } else if (!number->getUnit().isEmpty())
                 setQuantityProperty(key, number->getValue(), number->getUnit());
             else if(number->isInteger(&l))
                 setIntegerProperty(key,l);