Add missing styleable properties to ImageView?

[Johan Vos]

There is a discussion on an issue at github whether some properties on
ImageView need to be css-styleable:
https://github.com/javafxports/openjdk-jfx/issues/29

As not everybody is checking those issues, I want to bring this under the
attention of the list, as it has important consequences (once we do this,
what other properties might follow?)

There is a related PR at https://github.com/javafxports/openjdk-jfx/pull/30

I don't have a particular opinion on this topic, but I want to hear what
others think.

- Johan