Disable apply/update filter button when nothing is selected
Implement more robust checks for trivial context conditions
Fix logic for determining if a context is trivial to account for multiple children
Remove unnecessary null checks on context children since it is guaranteed
Make use ContextNodeModel.isValid for asserting validity

This previously required passing the `strict` option to only include
valid nodes in the output. However having a non-strict behavior has not
proven to be useful so it has been removed.

It was originally intended to allow serialization of the working nodes
to include non-valid nodes, but this is not being used anywhere in the
codebase.
Merge pull request #276 from cbmi/issue-270

Disable apply/update filter button when nothing is selected