PIVOT-946: With the addition of a GridPane.Row(GridPane) constructor, we need a
default no-arg constructor as well, which used to be supplied by the compiler
because there were no declared constructors.


git-svn-id: https://svn.apache.org/repos/asf/pivot/trunk@1589011 13f79535-47bb-0310-9956-ffa450edef68