gluon-web-model: add dependency support for sections

- Add a dependency array to section template
- Lift dependency handling from AbstractValue to Node
- Split resolve_depends() into a function handling dependencies of a
  node itself (resolve_node_depends()), which now is used for all nodes,
  and the existing recursive descent through the node tree
- When any node's dependencies are not satisfied, all descendents are
  reset recursively, meaning that an option in a section that is hidden
  will always be ignored, as if the option's own dependencies were
  unsatisfied

Fixes: #1349