Enhance the property metadata model

[Michael Strauß]

PR 1936 [0] proposes to introduce observable attached properties that
are distinguishable from instance properties with a naming convention
(this is also relevant for PR 1714 [1]). However, before doing this,
we should take a step back and see if we can't do better than that and
make attached properties more accessible in the property API.

I've prepared a document that explores a few options for doing so, and
a proposal for what I think might be a natural enhancement of the
existing property metadata model [2].

Comments are welcome.

[0] https://github.com/openjdk/jfx/pull/1936
[1] https://github.com/openjdk/jfx/pull/1714
[2] https://gist.github.com/mstr2/2fec0303fc440b8eaeb126befc76eb5c