[API-REVIEW] RT-27785: Add "margin" property to node and make it styleable from CSS

[Richard Bair]

As previously discussed, this patch adds a "margin" property to Node, bundled in the MiscProperties bucket so as not to incur overhead for people not using it. As it turns out, there is a non-obvious moment when MiscProperties will also get inflated -- if you call BorderPane.clearConstraints(node) (or clearConstraints on any of the other *Panes), because we have to check whether the property is bound prior to resetting it.

I'm not really happy with that, so if I can get a code review from Jasper maybe we can get something a little nicer worked out here.

Otherwise, pretty straightforward.

http://javafx-jira.kenai.com/browse/RT-27785