Missing methods in ObjectExpression and GridPane

[Martin Sladecek]

Hello,
these two issues were probably just oversights.

https://javafx-jira.kenai.com/browse/RT-25759

ObjectExpression is missing asString() methods, although other kinds of 
Expression objects have it.

https://javafx-jira.kenai.com/browse/RT-29848

GridPane is missing setFillWidth(Node, boolean) and setFillHeight(Node, 
boolean) static methods as otherwise this only possible for the whole 
row/column using RowContraints/ColumnConstraints.

Thanks,
-Martin