Styling HBox/VBox/GridPane child properties (hgrow, margin, columnIndex, etc)

[Michael Strauß]

This looks like a nice enhancement. The syntax is similar to CSS
features like Grid Layout  [0] ("grid-column" and "grid-row" are
properties on children of a grid container). We need to figure out how
these extension properties work with transitions. They should follow
the same transition rules as regular properties.

[0] https://www.w3.org/TR/css-grid-1/