[REVIEW] Support leading backslash as escape character in FXML

[Greg Brown]

>> This change would add support for a leading backslash as an escape character in FXML:
>> 
>> http://javafx-jira.kenai.com/browse/RT-18680
...
> I would vote to only support the new version and documenting in a
> migration guide that $$ is not supported any more but that's just me.

I think we'll want to maintain the existing syntax, at least initially, so apps don't suddenly stop working. However, I can certainly see an argument for deprecating the old syntax. Maybe we could display a warning in the console advising the developer to use the new syntax.