Corrected XMLElement.getChildren(name): was returning all descendants with that tag name

Corrected XMLElement.getChildren(name), because it was using method getElementsByTagName, which returns all descendants with that tag name, instead of returning only the direct children with that tag name.

In the corrected version, XMLElement.getChildren(name) will return only the direct children with that tag name.
Also created a new method called XMLElement.getDescendants(name), which returns all descendants with that tag name.