Improve property system to facilitate correct usage

[Michael Strauß]

I propose a set of changes to the JavaFX property system that I've
outlined in this PR: https://github.com/openjdk/jfx/pull/590

The changes fall into two categories: enforcement of correct usage
(there are several cases listed in the PR), and deprecating untyped
APIs (for removal in a future version) so as to make the intent of the
API more clear to developers.

Even though there are breaking changes, the impact on application code
should be minimal. I'd welcome any comments on this proposal.