======================================================
PropertyConstraintList is provided with the ability to check its constraints indices against
a minimum and a maximum, and set an invalidindex status.
In this status, the getValues returns an emptylist, as in the case with invalid geometry types.
===================================================================================
On changing the geometry property (for example from Python), the constraints geometry indices was not rebuild in order to avoid
redundant and unnecessary rebuilds. However, this might cause crashes, as the status of the sketch (or its properties) may be invalid.
It also refactors into OnChanged common functionality.
This commit does NOT solve that the user may be inserting invalid geometry indices to the First/Second/Third of Constraints (invalid input).
Only makes sure that geometry indices (geometry types) of PropertyConstraintList match the geometry.
Solution:
1. Force the rebuild of the constraint geometry indices upon assignment of new Geometry.
2. Force the rebuild of the constraint geometry indices upon assigment of constraints, if they result in invalid geometry indices.
3. Introduce the concept of internal transaction to avoid those rebuilds, checks and updates in case of an ongoing internal transaction,
thereby preventing them as it was done before introducing 1 and 2 (in the case of SketchObject internal transactions).
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.
