SCROLLGRID content is checked by fglform

When using a SCROLLGRID, fglform compiler checks that it does not hold other list containers.

Before version 3.10, fglform did not give an error when using TABLE, TREE or SCROLLGRID as sub-elements in a SCROLLGRID container, for example:

SCROLLGRID gr2
{
 <TABLE tb4   >
 [ab8        ]
 }
END

This layout construction is illegal: It can not be rendered by the GUI and causes undefined front-end behavior or crash.

Starting with 3.10, fglform gives a compilation error -6846 , if SCROLLGRID contains elements other than simple form fields or labels.