Issue 2507: Stop the entanglement of context properties and grob property internals

Issue 2507: Stop the entanglement of context properties and grob property internals

This introduces a semi-opaque structure Grob_properties meeting the
predicate ly:grob-properties? that is algorithmically handled at the C++
level via a wrapper structure Grob_property_info.

Encapsulating grob properties in that manner reduces the potential for
clashes and makes it easier to change algorithms and/or internal
representation.

While the principal distinction between context properties (one value
per context) and context-based grob property templates (one stack per
context) remains, at least the separation of the handling is more
pronounced.