Remove runtime check when constructing FunctionExpression, in order to
support future built-in function. Exception will be thrown when
evaluating the expression to inform user about the error.
* 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.
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.
