Change syntax validation processing, part 1

Add the following information in JsonSchemaTree:

* the list of checked paths;
* the list of unchecked paths;
* whether the schema is valid.

Add methods to:

* add a checked path;
* add an unchecked path;
* set the valid status (it is _false_ by default);
* query whether this tree has been validated at its current pointer.

Signed-off-by: Francis Galiegue <fgaliegue@gmail.com>