Controls like HBox, VBox and GridPane have several properties that can be set on the child via static accessors. They store these properties, like `hgrow`, `column-index` etc in the child's properties map under a specific key.

However, this makes it hard to style these controls from CSS. Settings like margins, grow and fill are typically choices that should be configurable from CSS.

Proposal:
- Modify Styleable to have a `getChildCssMetaData` method for providing CSS styleables applicable to any direct children
- Instead of storing key/value pairs in the properties map, store key/property pairs, where property is a styleable property created on demand
- Modify CssStyleHelper to take these additional styleables in to account